Can my opponent use Ruined Rex's Deathknell to kill the 2 Mechs that will appear from Ferrous Forerunner's Deathknell as a result of the same combat?

Question

Accepted Answer

No, your opponent cannot use **Ruined Rex**’s Deathknell to kill the two Mech tokens produced by **Ferrous Forerunner**’s Deathknell in this scenario.

Here is the breakdown of why this is the case based on the game's timing rules:

### 1. Triggering Order
Both Ferrous Forerunner and Ruined Rex trigger their Deathknell abilities simultaneously when they die during the Combat Damage Step (Rule 808.2). As the controller of those units, you choose the order in which these triggers are added to the chain (Rule 808.2.a).

### 2. Resolution and Healing
The combat sequence dictates that units are healed during the **Combat Cleanup** step, *before* the chain resolves (as established in **RiftJudge FAQ #10182** and **FAQ #10090**).

*   **Combat Damage:** Both units die. Their Deathknell triggers are added to the chain.
*   **Cleanup/Healing:** All surviving units on the board are healed.
*   **Resolution:** Only *after* the healing step does the game return to the FEPR (Finalize, Execute, Pass, Resolve) process to resolve the items on the chain.

### 3. Why the Mechs cannot be killed
Even if you order the chain so that Ferrous Forerunner's Deathknell resolves first:
1.  **Placement:** Ferrous Forerunner's Deathknell resolves, and you play the two Mech unit tokens to your base.
2.  **Targeting:** Ruined Rex's Deathknell trigger is already on the chain. Per the rules of targeting and resolution, if an effect requires a target, that target must exist when the ability is placed on the chain or be chosen upon resolution. 
3.  **Timing:** Ruined Rex's Deathknell trigger was added to the chain **at the moment of Ruined Rex's death** (Rule 808.1.d.2). Because the Mech tokens did not exist at the time Ruined Rex died, they were not valid targets for the trigger when it was created.

In short, Ruined Rex's Deathknell deals 4 damage to a unit that was present when he died. Since the Mech tokens were not yet in play, they cannot be targeted by that specific trigger.