Does Imperial Decree trigger the moment something takes damage?

Yes, Imperial Decree triggers the moment a unit takes damage.

According to the RiftJudge FAQs (#1917, #5341, #8633), Imperial Decree creates a lasting effect for the turn that functions as a triggered ability. When any unit takes damage while this effect is active, the trigger fires immediately.

Here is how the interaction works:

1. The Trigger Event: When a unit takes damage, the event of "taking damage" acts as the trigger condition.
2. The Chain: The trigger is placed on the chain. Because it is a triggered ability that uses the chain, it provides a window for players to play [REACTION] cards (such as flipping a hidden Zhonya's Hourglass) before the ability resolves.
3. Resolution: Once the chain resolves (or if no reactions are played), the effect executes, and the damaged unit is killed.

Important Nuances:

• Not Retroactive: As clarified in RiftJudge FAQ #2873 and #8633, Imperial Decree only triggers for damage taken after it has resolved. It does not look back to kill units that were already damaged before the spell was cast.
• Damage Event vs. State: It triggers on the event of receiving damage, not simply because a unit is currently in a damaged state.
• Combat Assignment: Imperial Decree does not change standard combat damage assignment rules (you must still assign full might to a unit before moving to the next). However, because it kills units upon taking any amount of damage, it is effectively used to clear enemy boards by sending units into combat sequentially.