If I hook a Ruined Rex, does the Deathknell or the Hook summon resolve first?

When you use Baited Hook on a Ruined Rex, the unit you play from your deck resolves first, and the Deathknell trigger resolves second.

Why this happens

According to the Riftbound rules for the chain (LIFO - Last In, First Out) and the specific interactions of Baited Hook:

1. Baited Hook Activation: You activate Baited Hook and target your Ruined Rex.
2. Resolution Begins: Baited Hook begins resolving. It kills your Ruined Rex.
3. Triggering Deathknell: Because Ruined Rex has Deathknell, its ability is created and added to the chain as a pending item immediately (Rule 808.1.d.2), before Baited Hook finishes its instructions.
4. Hook Continues: Baited Hook finishes its remaining instructions: you look at the top 5 cards and play a unit.
5. New Unit Enters: The unit you chose enters the battlefield and finalizes. If that unit has a "When you play me" ability, that ability is added to the chain as a pending item.
6. Resolution (LIFO): The chain now resolves in reverse order:
   • Any "When you play me" ability from the new unit resolves first.
   • The Deathknell trigger from Ruined Rex resolves last.

This is confirmed by RiftJudge FAQ #10390 and FAQ #8567.