Seal of Doom

The whole point of a Seal is to pre-pay the mana now and hold the kill for free: you cast it on a quiet turn, and three turns later the destroy effect costs nothing but the sacrifice. The Nemesis Seal cycle took five workhorse instants and sorceries and reframed them as enchantments that sit on the board threatening to go off, and this one is the cycle's hard creature removal: a Dark Banishing variant that strips regeneration but answers only nonblack creatures. The design's load-bearing trick is decoupling tempo from timing. The card resolves at sorcery speed when you cast it, but the activated ability that kills something can fire at instant speed, letting you leave up no mana and still hold removal. What you pay for that flexibility is transparency: an opponent who reads Dark Banishing in your graveyard learns nothing, but a Seal of Doom on the table is information they have to play around, and a savvy player can bait it or simply decline to commit their best creature into an open sacrifice outlet. The ability is not immune to interaction (it is an activated ability like any other, vulnerable to anything that counters one), but the spell that put it there is long resolved, so there is no casting window to disrupt. The nonblack clause is the real cost of the rate: the card is dead against the recursive black bodies a mirror leans on, which is exactly the matchup where a clean unconditional kill would matter most.