Dread Cacodemon

The clause that matters here is "if you cast it from your hand": a guardrail that closes off the reanimation and flicker tricks that make ten-mana demons playable in the first place. No returning it from the graveyard with Reanimate, no flickering it for a repeated board sweep. Cast it from your hand and the reward is a one-sided wrath stapled to an 8/8 body. Note the precise wording, though: the trigger asks where the spell came from, not how you paid for it, so casting it from hand without paying its mana cost (an Omniscience line, say) still arms the destroy-all-creatures clause. The follow-up (tap all other creatures you control) is the honest tax on that asymmetry. It does not clear a lane for an immediate alpha strike: the demon has no haste, and by your next turn your tapped board untaps anyway. What the tap actually buys is a window of vulnerability on your own side right after the wipe, a deliberate cost for resolving an effect that leaves you with the only sizable creature on the table. It is a finisher disguised as removal, or removal disguised as a finisher, depending on the board when you finally afford it. Black has long had cheaper, more flexible sweepers; the appeal here is the body that survives its own sweep, a single-card haymaker for a grindy deck once the mana is finally there.