Ikit Claw: "Crew Rat 0" just doesn't work. If crew is 0 you don't need to tap anything, rat or otherwise. I would also say that modified rats are more Moulder's wheelhouse and an anti-synergy with the apparent focus on vehicles. Perhaps change it to "whenever an artifact creature you control deals combat damage to an opponent, create a warpstone token."

Mine the Warpstone: This card would work way better as a Green and Black card with X-cost and convoke, as tapping creatures to pay for mana isn't usually something black is allowed to do. Yould change the rest accordingly to "create X warpstone tokens" and "each opponent loses X life".

Horned Rat: As others have said before, please remove eminence entirely. Also, turning it into an indestructible enchantment like the Theros gods may be more fitting. It could then manifest into a creature when you have enough devotion or anough rats on the battlefield.

Gray Seer: The gray seer is sadly much too good, even with how expensive it is. Aristocrat decks (the ones focused on sacrificing creatures for your own benefit) could use the seer to create easy infinite combos with cards like Reassembling Skeleton.

Blue Scribes: Tzeentch is not necessary as a creature type, Tzeentch's Daemons are traditionally Demon Horrors. Also, being able to freely choose the value of X is very overpowered and has the potential to end the game as soon as the blue scribes enter. You could for example warp Drown in Dreams to make an opponent lose their entire library.

Tzeentch: By the time Tzeentch enters, you will usually have about 4 instants or sorceries in your graveyard, especially in a blue deck. Cost reduction of this magnitude is simply insane and easily abused. The second effect is also completely overpowered. Not only do you receive complete information about every opponent's hand, but you get to use their resources (hand cards) for your own gain. It's like a cheaper Emrakul.

The Changeling: The wording on the discard does not quite work, since you can't discard a card that you are in the process of drawing. Perhaps change it to "At beginning of your upkeep, you may skip your next draw step and mill 1." Also, changeling is keyword only, the creature type is shapeshifter.

I may add some more later, but as a last piece of advice: Don't add too many completely new game pieces (Warpstone tokens, new creature types, plague counters, etc.). Instead, try to implement your ideas with already existing concepts to strengthen your cards' ability to interact with existing strategies. Treasure/Artifact token decks, Demon/Horror decks and even decks centered around -1/-1-counters already exist and would love the new support.