If my unit and an enemy unit die and both have Deathknell, what's the order for the triggers?

When multiple units with Deathknell die simultaneously (such as during a Cleanup step or a single game effect), their Deathknell abilities trigger simultaneously. These triggers are then added to the chain in an order determined by the players, starting with the Turn Player, and resolve in Last-In, First-Out (LIFO) order.

According to Rule 376.3.b.1:

"If multiple players separately control Triggered Abilities that are Triggered simultaneously, then starting with the Turn Player and proceeding in Turn Order, each player orders their Triggered Abilities on the Chain."

The Sequence:

1. Triggering: Both units die simultaneously, triggering both Deathknell abilities.
2. Ordering: The Turn Player chooses the order to place their controlled triggers on the chain first. Then, the next player in turn order does the same for their triggers, and so on.
3. Resolution: Once all triggers are on the chain, they resolve in LIFO (Last-In, First-Out) order, meaning the last trigger added to the chain resolves first.

Note on FAQ: As per the RiftJudge FAQ #4235, if both triggers are from the same event (the units dying), the controlling player(s) choose the order to resolve them. If you control both units, you choose the order for both. If you and your opponent each control one, the Turn Player chooses the order for their trigger first, followed by the other player.