When I play Sacrifice and kill my own Ruined Rex as the additional cost, does the Deathknell ability resolve before or after Sacrifice's draw and channel effect?

Ruling: The Deathknell ability resolves before Sacrifice's effect. When you kill Ruined Rex as part of Sacrifice's additional cost, the Deathknell trigger becomes a pending chain item that finalizes after Sacrifice finalizes. Because the chain resolves in Last In, First Out (LIFO) order, the Deathknell (newest item) resolves first, dealing 4 damage, then Sacrifice resolves, drawing 2 cards and channeling 1 rune exhausted.

Sequence:

• Play Sacrifice and kill Ruined Rex as the additional cost during finalization
• Ruined Rex's Deathknell becomes a pending chain item
• After Sacrifice becomes a finalized chain item, Deathknell finalizes and is placed on the chain as the newest item
• The chain resolves: Deathknell first (deal 4 damage), then Sacrifice (draw 2, channel 1 exhausted)

Nuances:

• Pending items finalize up the chain, but finalized items resolve down the chain (newest to oldest)