B · Engine & Sequencing · §9
The kill is an additional cost — uninteractable, with the Deathknell resolving before the draw.
Sacrifice kills a friendly Mighty unit as an additional cost. That distinction creates three major advantages over Hook.
The unit dies while costs are being paid. By the time the opponent receives a normal reaction window, the body is already in the trash. If Sacrifice is countered, the killed unit remains dead.
The sacrificed unit’s Deathknell is placed above the spell and resolves before drawing two and channelling the exhausted rune.
When an opponent targets one of your Mighty Deathknell units, Sacrifice can convert it immediately. Strongest examples: Ruined Rex targeted by removal; Rift Herald about to be neutralized; Dragon Form applied to a small Deathknell body; Glasc Mixologist after it has contested a battlefield; a five-Might Reflection that will die to Temporary anyway.