I don't mean to butt in, but feel an additional comment on Anuttymous' Infertile Ground is needed:
An enchantment with "Enchant tapped land" would be put into the graveyard as soon as the land untapps. (cf.: Entangling Vines)
To gain the desired effect a clause like on Dream Leash is needed.

I hope nobody minds me using this thread to point that out, but I felt like some additional rules-knowledge won't hurt if the design space that goes with it is explored.
If anyone does, feel free to delete this post. Thank you.

Arcane Sight - Not bad, it looks interesting, but splice was before my time so I've never used the mechanic.

Lord of the Labyrinth
Creature - Minotaur
Flash
Creatures don't untap.
Prevent all combat damage.
: Target creature attacks this turn if able.
2/4

New challenge - Create a mythic rare angel that costs only one white mana.

Splice onto --- has to splice onto a subtype, hence Arcane, otherwise Instant/Sorcery would have to be a subtype. I think, anyway.

Smooth, thanks for pointing that one out - I was planning on putting it in a set at some point, and it wouldn't have worked otherwise. Good job.

Lord of the Labyrinth 4/10 - Sorry, but a blue minotaur? I know that the abilities are blue, but generally you ca override the colour for flavour purposes, simply by upping the cost, or weakening the effect - see Hornet Sting and Entangling Vines. Also, flash seems completely unnecessary. 2/4 seems a bit weak for a Minotaur as well, but considering "Prevent all combat damage" it wouldn't matter.

Serra's Chosen
Creature - Angel {M}
Flying
As long as you control three or more white creatures, CARDNAME gets +4/+4 and has vigilance and lifelink.
1/1

Yeah, it could easily be second turn 5/5 scary, but look at Demon of Death's Gate, that could be second or third turn HUGE. And they're both mythic.

Next challenge: Make an angel equivalent of Demon of Death's Gate.

Serra's Chosen- 8/10. Something about it doesn't seem "Mythic" to me. Still, I like the design of the card.

Angel of the Golden Throne
Creature- Angel
You may exile six untapped white creatures you control instead of paying Angel of the Golden Throne's mana cost.
Flying, lifelink
An infallible judge, an unswayable jury, and a merciless executioner.
9/9

NEXT: Create an enchantment creature.

Yea, I did intend to have it Splice onto either Instant or Sorcery. Didn't expect to stumble into any rules issues, but I believe it's widely assumed that Splice could easily be modified to work with non-permanent supertypes.

Angel of the Golden Throne - 10/10 - Captures the essence nicely, and it is very religous, as a mass of people are giving up their lives for a "greater good".

Protector of Heaven

Enchantment Creature - Human Cleric
Other enchantments you control have shroud.
Other creatures you control have shroud.
“I stand between you and Heaven. Do you have the beauty to pass these gates?”
1/4
Rare

Next Challenge - Create a non-blue Ally with Flash with a reasonable "enters the battlefield" ability.

Protector of Heaven - 10/10. I wish I could give some constructive criticism on this one, but I'm afraid, I love it.
Flavorful, accurately costed and a very good way to bring in the duality of the supertypes. Kudos.

Sejiri Ambusher
Creature - Human Scout Ally
Flash
When ~ enters the battlefield, you may cast Ally cards as though they had flash until end of turn.
"Ever ambushed a cautions gladehart in the middle of a coverless steppe? No?
Well I guess there's always a first time. I'll take the lead."
1/1

Next:
Scars of Mirrodin Spoiler:

10/10 - Very very nice.

Make me a... Legendary Eldrazi, a 4th titan, but make it a 1/1 and give it some flavor on why the eldrazi has such weak p/t.

Keep in mind it need Annihilator and a "when you cast effect" and it has to be mythic.

Mox Opal - 10/10. The balance on this actually counters the others.

Equillus, He Who Remained
Legendary Creature - Eldrazi Mutant
1/1
When you cast Equillus, He Who Remained, you get an emblem with "Creatures you don't control are 0/1 creatures with no abilities."
Equillus, he Who Remained is indestructible.
Shroud
Annhiliator 2

Next - Create a new Phyrexian war machine. It must be a black aritfact.

Equilus: 9/10. Peculiar dude... the main reason for not getting 10/10? The flavor seems a bit off to me. Not sure how to describe it, save that I somehow feel the Eldrazi wouldn't either A) Prey on their brethren or B) Negotiate and then let it stay alive. Plus, how come he hasn't nommed all of Equilor by now?

SOM Spoiler!

Or the non-spoiler version...
Phyrexian Foegorger
Artifact Creature - Construct {R}
, : Destroy target nonblack, nonartifact creature. You lose life equal to its converted mana cost.
4/4

Next: Create a PW for Tezzeret the Remade [after he was beaten by the Kamigawa Nezumi but reconstructed by Bolas]

Tezzeret the Remade

Planeswalker - Tezzeret
Whenever an artifact card is put into your graveyard from anywhere, you may exile it. If you do, put a loyalty counter on Tezzeret the Remade.
+1: Search your library for an artifact card, reveal it, and put it into your hand. Then shuffle your library.
-3: Return up to one target artifact card in a graveyard, and put it onto the battlefield under your control.
-5: Exile all artifact cards in your graveyard. If Tezzeret is in a graveyard, return it to the battlefield with loyalty counters equal to the number of artifact cards exiled. Otherwise, put that many loyalty counters on him.
3
Mythic Rare

Three abilities plus a static. I designed the abilities based on the sample chapter for Test of Metal that came in the Core 2011 Fat Pack.

Next Challenge - Venser, Shaper Savant became a planeswalker at the end of Future Sight. Create a planeswalker card for him. See spoiler tag for disclaimer. (Scars of Mirrodin spoilers)

Tezzeret the Remade: Lets see...

First ability: 10/10, I like this one. Pretty cool, and pretty simple.
Second ability: 6/10, Also very simple, but way too strong. In the right deck, this is a reuseable Diabolic Tutor, on a + ability. No good.
Third ability: 7/10 Fine, minor grammar, but good enough.
Fourth ability: 5/10 I like the effect of it, but it isn't so good (In most cases, it will put him on less than 5 loyalty counters), and it doesn't have any synergy with any of his other effects whatsoever.

Overall: 7/10

Next challenge: More SoM spoilers!

A review of the previous card:

Hmm... I don't see much reason for spoiler tags for the rating, but just in case:

::EDIT:: This now fits the original challenge ::/EDIT::

Loom of the 3 Fates
Enchantment {R}
Sacrifice ~: Draw three cards.
When a noncreature permanent becomes the target of a spell or ability an opponent controls, put a fate counter on ~.
When ~ is put into a graveyard from the battlefield, you may draw three cards for each fate counter on it.

This card is meant to represent the 3 Fates of Greek myth, especially in the sense that while you could tamper with fate, it would just make matters worse.

Next Challenge: Create another card that uses the Ancestor mechanic like Vengeful Firebrand.
Ancestor - As long as a [type] card is in your graveyard, [effect]

Wurmcoil Engine: 10/10 very fits the graph and mirrodin, good work!

Abyss Grimoire
Artifact
When any permanent leave out battleground, exile it, then cast a spell from your hand without pay cost and lose half your life, rounded down.

Next challenge: Design a uncommon card that looks like broken, but it isn't, and really useful.

palewalker, you responded to the wrong challenge. Also, the last challenge was posted three days ago, so it's kinda hard to claim that you had been ninja'ed

Loom of the 3 Fates: I really like it, and it des what it is supposed to do. 9/10

Phyrexian Necrolyte
Creature - Zombie Carrier
Trample
Ancestor - When ~ comes into play, if a zombie card is in your graveyard, you may have target creature get -4/-4 until end of turn.

I dunno if it fits the challenge, but i think it does.

Next - Make another land in the style of Reflecting Pool or Exotic Orchard.

Phyrexian Necrolyte - 5/10 I do like it, it works great with zombies feel and black, but... where is the power/toughness?

Land of Tie-Dye

Land

: Add to your mana pool one mana of any color than an opponent is wearing.

"That's red."
"It's more of a pink, actually."

---------------

Next challenge, make a card called God of Goblins

Land of Tie-Dye: 7/10 -> It would have gotten more if it had been better worded. Moreover, what happens when an opponent is wearing purple and pink? Or gray and gold? 'twould be useless.

God of Goblins
Creature - Avatar Spellshaper {R}
Goblin offering (You may cast this card any time you could cast an instant by sacrificing a Goblin and paying the difference in mana costs between this and the sacrificed Goblin. Mana cost includes color.)
, , Discard a card: ~ deals damage equal to the number of Goblins you control to target player.
5/4

Basically, it spellshapes Goblin War Strikes.

Next challenge: Create a card that cares about +1/+1 counters in a way we haven't seen before. (e.g., no Bramblewood Paragons or [card]Gilder Bairn[/cards)

God of Goblins: I love it, but it may be a bit too powerful, with a cheap cost (thanks to offering) and repeatable damage (even if it's not a goblin). 8/10
About land of tie-dye, I think that silver-bordered cards can produce mana of any color, even pink or purple.

Avatar of Decay
Creature -- Avatar
~ costs less for each +1/+1 counter among permanents you don't control.
Each other creature gets -2/-2 for each +1/+1 counter on it.
6/6

Next challenge: a black and/or red aura with totem armor.

Avatar of Decay - That's a very interesting ability. A bit too situational for my taste, but good (and balanced) nonetheless. 8/10

Twilight Umbra
Enchantment - Aura
Enchant creature
Enchanted creature gets +1/+1 for each black mana symbol in its casting cost.
Totem armor (If enchanted creature would be destroyed, instead remove all damage from it and destroy this Aura.)

Magma Umbra
Enchantment - Aura
Enchant creature
Enchanted creature gets +2/+0 and has "Whenever this creature blocks or becomes blocked by a creature, it deals damage equal to its power to that creature."
Totem armor (If enchanted creature would be destroyed, instead remove all damage from it and destroy this Aura.)

Next up! Give me a black dragon that has Wither.

Twilight Umbra: 7/10: a little broken... but ability is very funny, it just need more cost.
Magma Umbra: 10/10 it is very good and balanced, great design.

Marxram, the doom
legendary creature - Dragon Avatar
When Marxram, the doom into battleground, reveal your hand cards and five cards from your deck top, then exile all creatures that reveal by this way.
Flying
All your creatures get Wither.
If you cast Marxram, the doom with more than three , all creatures get -2/-2.
8/8

Next: make a 4cc common card that really useful for Spike (plays to win) and Timmy (player likes win big), but for Johnny (player likes to win with style) it's a trash.

@palewalker - I'm just wondering, but what's funny about the ability for Twilight Umbra?

I said funny is mean good and interesting, no offend.

Marxam - 6/10 - Flavorful mana cost, but a ton of language issues, which I can retemplate simply by reading to get the jist of various effects. The last one though is weird, and a very dangerous memory issue. This is why the Volvers had different counter added based on which kicker was added.

Nightreed Stalker

Creature - Crocodile Warrior
Flash
When Nightreed Stalker enters the battlefield, you may have it block target attacking creature.
When Nightreed Stalker enters the battlefield, if it is the declare blockers step, it enters the battlefield tapped and atatcking.
4/3
Rare

Next challenge - Assume Jinxed Idol and Jinxed Ring are part of a block cycle. There was no Jinxed card in Exodus, so, finish the cycle.

Nightreed 8/10 I like it, but It just doesn't feel too green to me.

-----------

Jinxed Necklace
Artifact
If a source other than Jinxed Necklace would deal damage to you, Jinxed Necklace deals 1 damage to you.
Sacrifice a creature: Target opponent gains control of Jinxed Necklace.

-----------

On another note, I would see Twilight umbra as this: "Enchant Vampire. Enchanted vampire is now a foil card."

-----------

Next challenge, make an instant spell that costs 1 colourless mana.

That was the most awesome joke evar. Mind if i put that quote in my signature?

Jinxed Necklace - This actually doesn't seem to be something that needs to be jinxed; for 2 mana, +1 damage to opponents from all sources doesn't seem that good when compared to things like Honor of the Pure (even though this can go into any color deck). Of course, there's the fact that two of these will strangle someone to death instantly...

And as for the challenge...

Invoke All
Instant [Common]
Invoke All is all colors.
Spellmorph (You may cast this face down as a 2/2 creature for . You may cast it for its spellmorph cost by turning it face up.)
Add one mana of any color to your mana pool. Exile Invoke All.

Wait a minute... that card seems pretty useless, doesn't it?
Anyways, the next challenge: Make a card that has the Trace pseudokeyword, as such:

_____ [mana cost]
Instant/Sorcery
[Effect]
Trace <name> — If a <name> spell was cast this turn, [additional effect].

Alternatively, as an ability on a permanent, for example:
Trace <name> — [Cost]: [Effect]. Activate this ability only if a <name> spell was cast this turn.

(The keyword parameter is most often a color, but a card type or creature type can work too.)

Invoke All: 8/10 , high flexibility and simple, I like it.

Angelic echo
Instant
Target player gains 5 life.
Trace white — If a white spell was cast this turn, prevent all damage that would be dealt to you or your creature until end of turn.

next challenge: Make a 3cc Sorcery and show your style!

Um, the Necklace is doing that damage to you unless you can get rid of it. Hence the jinx.

Angelic Echo: 8/10. Fits the challenge, but the trace effect is powerful for the casting cost.

All Flash

Sorcery
Exile target creature that dealt damage to you this turn.
"People like you," remarked Dezora, "have no substance. Let me show you what I mean."

New Challenge:
Create a permanent that can be a temporary version of itself.

Yes, I meant that the jinxed card could be better off as an Enchant player-type thing, that can't be returned back to yourself.

All Flash- 4/10. It's all very good until you realize that most creatures deal damage to you on their turn, and this is a sorcery. If this was an instant, it would be okay.

My card is admittably uninspired, but...
Unstable Updraft
Enchantment
Echo
Creatures you control have flying.

Next challenge: make a card with the name of a movie or video game soundtrack.

Unstable Updraft: 6/10, the cost of echo is too much, but at least it fits the challenge.

One-Winged Angel
Creature - Angel
First strike
:One-Winged Angel gets flying until the end of turn.
He always look the sky where once his homeland.
2/2

next challenge:
Choice a card that you like from Homelands and fix it!

One Winged Angel: I was expecting that soundtrack! I think it's good, looking at other 2/2 creatures with WW mana cost and first strike.

Timmerian Fiends
Creature -- Horror
Protection from black
When Timmerian Fiends enters the battlefield, exchange control of Timmerian Fiends and up to one target artifact an opponent controls. If you don't make an exchange, sacrifice Timmerian Fiends. (This effect lasts indefinitely.)
3/2

Next challenge: make a card named "Chrono Trigger", OR make a card related with the videogame with the same name (example: a character, a place, etc.).

Timmerian Fiends Redux: Good choice with protection from black, sneaky way to make it that much more of a threat. Cool card that reminds me of Sleeper Agent.

Chrono Trigger -
Enchantment -
Whenever a player casts an instant spell, you may sacrifice Chrono Trigger. If you do, copy that spell. You may choose new targets for the copy.

I've never played the game, so this is completely based on the name.

You forgot to post a challenge.

Ah, I did! One sec ...

Next Challenge: Make a blue sorcery that costs either or . You're pick.

Chrono Trigger: I like it. It fits the name well, although it would probably be better costed at since Twincast only costs and putting effects on the table to save for later typically only costs more. As a sidenote, I haven't played Chrono Trigger either; all I know is that it involves time travel and cause-effect relation manipulation.

Time Freeze -
Sorcery -
Kicker—Sacrifice two Islands.
Target player skips his or her next turn.
If Time Freeze was kicked, it can't be countered by spells or abilities.

Next challenge: Make a , or rare Horse creature as though Nightmare and Thundermare were the first two cards of a cycle. (A very loose cycle, perhaps...?)

Timbermare <Even through it costs less, the kicked cost is kinda meh... Well, i guess someone can remake it.

Time Freeze: 8/10 simple and useful, but I think it shall be a rare not a mythic... anyway, nice work.

green is already had its mare, so here is white and blue

Tidemare
Creature- Elemental Horse
Flash
When Tidemare into the battleground, tap all other creatures.
4/5

Radiancemare
Creature- Elemental Horse
When Radiancemare into the battleground, tap all other creatures, they can't be untap on their owner's next untap phase.
It's different between hope and wish.
5/5

Next: How big can a 1cc creature to be? Make sure it wont be broken, and useful.
I guess need to change another...
Make a card that againsts Blightning

Tidemare- Flash makes this guy a handy, no-strings-attached blocker when the rest of your dudes are off fending Vengevines and Bloodbraid Elves intending to tear you a new one. Good job.
Radiancemare- Weaker than the Tidemare, and too similar to it. Give it some other ability.

Cura
Instant
Target player gains 5 life and draws two cards.
Especially skilled White Mages can heal wounds of the mind with a single spell.

NEXT: Take your favorite card and remake it in the spirit of Planar Chaos' "planeshifted" cards.

Cura - 8/10. A solid anti-Bligthning, but maybe a bit OP.

Holy Verdict

Instant
Destroy target creature. Itn can't be regenerated.

This is just the "planeshifted" Terminate.

Next - Create a green instant or sorcery card with a Graveboon ability (e.g. Graveboon - As long as ~ is in yopur graveyard, [effect])for my upcoming Legends of Colfbrand set.

Holy Verdict - Cool design. I'm uncertain about including blue, but I think it fits.

Fertile Graves -
Sorcery
Search your library for a land and put it into play tapped.
Graveboon - As long as ~ is in your graveyard, Green creatures you control have ": Add to your mana pool."

Next Challenge: Make a card that contains ten out of these thirteen words: Sorcery, creature, security, kavu, silver, carbon, coal, rise, terrify, door, crossbow, wave, darkness. The words can be anywhere on the card, even hidden within other words.

@windandfire - Can I use Fertile Graves in LOC?

Sure! Go ahead and change anything you need as well.

Coal Kavu 1RB
Creature - Elemental Kavu [U]
When ever you play an Instant or sorcery spell deal 3 damage to target creature if this creature is destroyed this way its controller looses 3 life
-Security!! The terrifying Carbon Kavu of darkness just broke down a silver door!!-

Next challenge make a cycle of encntment cards that are like leylines that utillizes mana

Don't forget to rate the previous card. A lot of people who tackle these challenges do so for feedback on their card creation skills.

Okay I think 6/10 'cause he is kinda cool having creaure giving creatures lannie effects

6/10 but no negative reasons given? Um...

Coal Kavu: 4/10. Got the challenge, though some of the words feel forced. It has no P/T and that ability as written will only trigger if it's the only creature on the board. And it's LOSES.

Now... yay, a cycle. o_o

Bolstering Channels

Enchantment
Any time you pay any amount of white mana, creatures you control get +0/+1 until end of turn.
The energy flowed through the soldiers, and they fought as though they had no wounds.

Refreshing Channels

Enchantment
Any time you pay any amount of blue mana, you may untap a tapped creature you control.
Tired merfolk had been coming to the nexus for years, though they could not say why.

Paralyzing Channels

Enchantment
Any time you pay any amount of black mana, you may tap up to three target creatures Each creature tapped in this manner deals 1 damage to its controller.
When all you had is gone, all that's left is pain.

Blistering Channels

Enchantment
Any time you pay any amount of red mana, you may put a pain counter on target creature. Creatures with pain counters get -1/-1 and cannot attack.
One little sore, and you can easily forget it. One thousand, and you can easily forget anything else.

Invigorating Channels

Enchantment
Any time you pay any amount of green mana, regenerate target creature.
The elves of the Ahrean Wood were notoriously difficult to defeat.

You think it's easy, and then...

Challenge: Design a non-red card with a normally mono-red mindset.