Demagog, you don't want Plagues in a deck with Carcinogen.
Realistically, this could be considered the game's only true "kill a creature" spell. Sure, it gives rainbow decks plenty of Feral Bond fodder, but it's the only spell that doesn't take an enemy creature's toughness into account at all when removing it from the game.
That alone is worth quite a bit. Colossal Dragon? Not Anymore!
The slot-filling thing is just evil.
Mark of Gravity
14 Death Tower
5 Carcinogen
5 Deadly Poison
3 Elite Otyugh
3 Bone Wall (Elite)
Tight, evil, and very hard to deal with. Save up 7 Death mana ASAP, and hit the first thing on the board with Carcinogen. Then sit back and use Oty+BoneWall on Cancer Cells for defense, and ever-growing Otys+Deadly Poison for offense.