Does Ruined Rex Deathknell damage happen before or after cleanup in a Showdown?

In a combat showdown, Ruined Rex's Deathknell ability deals its damage after the healing step of the Combat Cleanup.

While the trigger for the Deathknell is identified and added to the chain during the Combat Cleanup, the actual execution of the effect happens only after the healing step is completed, as part of the game's finalization process.

The Combat Cleanup Sequence

According to the official rules governing combat (specifically Rule 323.4, Rule 808, and the clarifications in RiftJudge FAQ #9786 and FAQ #10018), the process is as follows:

1. Combat Damage Step: Combat damage is dealt to units (Rule 460.2.d).
2. Triggering Deathknell: Units with lethal damage trigger their Deathknell abilities. These triggers are added to the chain as Pending Items (Rule 808.1.d.2).
3. Killing Units: Units with lethal damage are killed and placed in the trash (Rule 323.5).
4. Healing: Surviving units on the battlefield are healed (Rule 461.1.a.1).
5. Resolution: After healing is complete, the game returns to the FEPR (Finalize, Execute, Pass, Resolve) process, and the Deathknell triggers on the chain are resolved (Rule 808.1.d.3).

Because healing occurs in Step 4 and the resolution of the Deathknell occurs in Step 5, Ruined Rex’s damage cannot be used to "finish off" a unit that survived the combat damage, as that damage will be cleared by the healing step before the Deathknell effect executes.