Death-Rattle Oni

The two halves of this card are a trap laid for an opponent who thinks in terms of the board in front of them. A seven-mana demon that flashes in is nobody's plan; a demon that shaves off its cost for every creature that died this turn, dropped at instant speed in the middle of combat, is a different animal entirely. The cost reduction rewards you for turning a board into a graveyard first: block, trade, chump, or resolve a sweeper, and the price collapses. Then the enter trigger closes the loop by destroying every other creature dealt damage this turn, so a single well-chosen combat step becomes a one-sided wrath that also leaves a 5/4 standing on the wreckage.

Flash timing is what makes that trigger vicious rather than a plain sorcery-speed board wipe. Because "dealt damage this turn" counts damage marked but not lethal, you can hold the demon until after combat damage is assigned, let creatures survive a combat they would otherwise walk away from, and then destroy them anyway before the cleanup step wipes the marks clean. A blocker that survived a smaller attacker, a creature clipped by an earlier burn spell, anything carrying marked damage it lived through: all of it becomes eligible. The design asks you to read a full turn ahead rather than react to the current board, and it punishes an opponent for the very act of committing creatures to a fight.