Does Frozen Fortress work with Imperial Decree?

Yes, Frozen Fortress works with Imperial Decree.

If Imperial Decree is active, the 1 damage dealt by Frozen Fortress during the Beginning Phase will trigger Imperial Decree, causing those units to be killed.

Reasoning

1. Damage Source: Frozen Fortress deals 1 damage to each unit at the battlefield at the start of the Beginning Phase.
2. Trigger Condition: According to RiftJudge FAQ #9789 and #8420, Imperial Decree creates a triggered ability that monitors for the event of a unit taking damage. It triggers on any damage dealt to a unit, regardless of the source.
3. Interaction: When the Frozen Fortress ability resolves and deals 1 damage to a unit, the event of "taking damage" has occurred. This satisfies the condition for Imperial Decree, which then triggers and kills the damaged unit.

Sequence

• Beginning Phase: The Beginning Step begins.
• Frozen Fortress: Its ability triggers and resolves, dealing 1 damage to units at its battlefield.
• Imperial Decree: Because a unit took damage, the Imperial Decree trigger is placed on the chain.
• Resolution: Once the chain resolves (or if no reactions are played), the effect executes, and the damaged unit is killed.

As clarified in RiftJudge FAQ #9882, the Beginning Phase occurs before the Scoring Step, so this sequence happens before you proceed to score Victory Points for holding battlefields.