Thousand-Tailed Watcher attacks my Ruined Rex at a battlefield. I deal 6 damage to Thousand-Tailed and he kills my Ruined Rex. Does my Deathknell resolve dealing 4 damage to the Thousand-Tailed, killing the Thousand-Tailed that has 6 damage from combat? Or is there combat cleanup before my Deathknell resolves? Please explain.

No, your Ruined Rex's Deathknell will not kill the Thousand-Tailed Watcher in this scenario. The Watcher will survive the combat.

According to the official rules and RiftJudge FAQs, healing occurs during the Combat Cleanup, which happens before your Ruined Rex's Deathknell ability resolves.

The Combat Sequence

Here is the step-by-step breakdown of your showdown:

1. Combat Damage Step: Combat damage is dealt. Your Ruined Rex (6 Might) and the Thousand-Tailed Watcher (7 Might) deal damage to each other.
   • Your Ruined Rex is marked with 7 damage (lethal) and dies.
   • The Thousand-Tailed Watcher is marked with 6 damage (not lethal, as its Might is 7).
2. Triggering Deathknell: As Ruined Rex dies, his Deathknell ability triggers and is added to the chain as a pending item (Rule 808.1.d.2).
3. Killing Units: Ruined Rex is moved to the trash (Rule 323.5).
4. Healing: Surviving units on the battlefield (including the Thousand-Tailed Watcher) are healed (Rule 461.1.a.1). All damage marked on the Watcher (the 6 damage from your Rex) is cleared.
5. Resolution: Only after the healing step is complete does the game return to the FEPR (Finalize, Execute, Pass, Resolve) process to resolve the pending Deathknell trigger on the chain (Rule 808.1.d.3).

Conclusion

Because the 6 damage on the Thousand-Tailed Watcher is removed during the healing step (Step 4), the Watcher is at full health when your Deathknell effect finally resolves (Step 5). Dealing 4 damage to a 7-Might Watcher that has already been healed to full health is not enough to kill it.

Sources: RiftJudge FAQ #10018, FAQ #10090, and FAQ #10634.