If I attack a Trifarian War Camp battlefield that my opponent has a Ruined Rex on with my Leblanc Fragmented, then ambush in with a Vi Peacekeeper, would the Deathknell effect from Ruined Rex be able to kill my Leblanc?

To determine if the Deathknell effect from Ruined Rex can kill LeBlanc, Fragmented in this scenario, we must examine the sequence of the combat showdown and the timing of the healing and Deathknell resolution.

The Short Answer

Yes, the Deathknell effect from Ruined Rex can kill your LeBlanc, Fragmented.

While the healing step clears damage sustained during the combat damage step, it does not prevent the Deathknell from dealing new damage to LeBlanc once it resolves. If the 4 damage from the Deathknell is directed at LeBlanc, she will be at risk regardless of whether she was healed from prior combat damage.

Detailed Breakdown

Based on the combat flow in Riftbound:

1. Combat Damage Step: Combat damage is dealt. If your unit is dealt damage equal to or greater than its Might, it is marked with lethal damage.
2. Deathknell Triggering: As Ruined Rex dies in combat, his Deathknell ability is added to the chain as a pending item (Rule 808.1.d.2).
3. Killing Units: Lethal units are moved to the trash.
4. Healing Step (Combat Cleanup): Per RiftJudge FAQ #10090, FAQ #10182, and FAQ #10545, all surviving units on the battlefield are healed during the Combat Cleanup step (Rule 461.1.a.1). Any damage they received during the combat damage step is cleared.
5. Deathknell 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.

Because the healing occurs in step 4 and the resolution of the Ruined Rex's 4 damage occurs in step 5, the Deathknell is applied to a unit that has already been cleared of combat damage. If the Deathknell damage is sufficient to match or exceed your unit's current Might, it will kill the unit at that time.

Note: I could not fully verify this ruling against the official retrieved rules regarding the specific Might stat of LeBlanc, Fragmented.