MTG Salvation

Horrid Contortion (G/P)(U/P)(R/P)
Instant U
Spend at least one colored mana to cast Horrid Contortion.
Target creature loses all abilities and becomes a colorless Eldrazi with base power and toughness 4/4. At the beginning of the next end step, that creature's controller sacrifices it.
The Mirrans had thought they had seen the worst conceivable horrors during the war against Phyrexia. Then the Eldrazi came.

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I feel like the first ability is just there to meet the challenge. If it weren't for it, I think you would have very probably just used colored mana in the mana cost. Losing all abilities and transforming into an Eldrazi feels blue to me, and the sacrifice at end of turn is red. I honestly don't see any green here. I would have probably made this card just blue-red. No problems with the flavor, but let me just say that the Mirrans are so unlucky! First Phyrexia, then the Eldrazi! I feel like the famous "Curse guy", the "unluckiest planeswalker in the Multiverse" is nothing compared to the bad luck of the Mirrans here!

Radix Raptor (G/P)(G/P)
Creature - Bird Horror (Rare)
Flying
(G/P), T: Add one mana of any color.
(W/P)(U/P)(B/P)(R/P)(G/P), T: Proliferate, then proliferate again.
Each feather in its tail is a shimmer of another world, waiting for Compleation.
0/1

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It's very hard to find something I don't like here. The only minor remark I might have is wondering if all five colors are actually needed to proliferate twice, but I admit it's very elegant this way. I'm also a big fan of the mana ability being either mana acceleration or just filtering depending on how you choose to pay the Phyrexian mana. Very good job.

Cerebral Seizure (U/P)(B/P)
Sorcery (U)
Target player discards a card at random.
Draw a card.
"Pardon me while I pick your brain."

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I love the simplicity and elegance of this card, but I would honestly hate it being played against me, but that's actually true for all "discard at random" effects. I also feel like this could just be a monoblack cantrip spell, even if it's certainly not out of the color pie as is. Overall a very solid job, but I personally would like this card much better without the words "at random", increasing the mana cost accordingly.

The Uniter 7
Legendary Artifact Creature (Mythic)
Creature spells you cast cost 1 less to cast for each 1 life you paid this turn.
: Put a -1/-1 counter on target creature, then move all counters from one permanent onto another with the same type, then proliferate.
3/5

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There are several Quality mistakes here. The first 1 should be a mana symbol. The order of mana symbols in the activation cost is wrong: now the Tarkir order is the only official order for wedges (see an "Oracle changes" articles for a recent set that I don't remember, maybe Dominaria? I might check this later), so it should be black-green-blue here. Finally, check Fate Transfer for the wording of the last ability: it should be "...move all counters from a permanent onto another permanent that shares a card type with it..."
I like what the card is trying to do in itself, but it's honestly a bit too complicated for my taste. Good that it's a mythic for that.

Prison Realm Invasion 2(U/P)(U/P)(B/P)(B/P)(R/P)(R/P)
Sorcery (MR)
Exile the top four cards of you library, then put a creature or planeswalker in exile or in a graveyard onto the battlefield under your control. It deals damage equal to its converted mana cost to up to one creature or planeswalker.
Those who thought the multiverse would never see a threat like the God-Pharaoh Bolas again were not prepared to meet the new Father of Machines.

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Quality mistakes, this should be:
The first is a typo. The second is just needed, "creatures" and "planeswalkers" are called so only on the battlefield, both in exile and in graveyards there are "creature cards" and "planeswalker cards". The third one might be needed depending on how you want the card to work technically.
About the card itself I have two things to say. First, I really don't like that this card can return things from exile. It's maybe the only thing left to avoid exile being just a second graveyard. I don't think we should make the line between graveyard and exile even thinner than it already is. Second, the flavor of this card really intrigues me. It makes me wonder who the new Father of Machines is. A flavor text that leaves you with the curiosity to go deeper in the story is a good thing.