If I use Imperial Decree and attack a battlefield with a Sand Soldier vs a unit with 3+ Might and they're playing Draven legend, when does Imperial Decree kill the unit? Does Draven win the combat?

This interaction is governed by the timing rules of combat and the resolution of triggered abilities.

Ruling

The Draven player will draw a card from their legend ability before the unit is killed by Imperial Decree.

Even though Imperial Decree's effect is active, the "kill" trigger does not resolve until after the combat winner has been determined. Because the combat winner is determined while the Imperial Decree trigger is still pending on the chain, the game state still sees the units as present, allowing Draven's "When you win a combat" ability to trigger and resolve first.

Sequence of Events

1. Combat Damage Step: Combat damage is dealt simultaneously.
2. Triggering: Imperial Decree's condition is met (the unit took damage), and its "kill" trigger is added to the chain as a Pending Item.
3. Pending State: The trigger cannot resolve yet because the current game effect (the Combat Damage Step) must finish completely first.
4. Combat Winner Check: The game checks the battlefield state to determine the winner of the combat. At this moment, the Imperial Decree trigger is still on the chain and has not resolved, so the units that survived the combat damage are still on the board.
5. Draven's Ability: Because the opponent's unit is still on the board (and yours is as well), the game evaluates the combat result. If your unit remains and theirs does not (or vice versa), the winner is determined. Draven's "When you win a combat, draw 1" ability triggers and is added to the chain on top of the Imperial Decree trigger.
6. Chain Resolution (LIFO): The chain resolves using Last In, First Out:
   • First: Draven's draw ability resolves.
   • Second: Imperial Decree's "kill" trigger resolves, removing the damaged unit from the board.

Nuances

• Timing Window: There is a specific timing window between damage being dealt and the unit dying to Imperial Decree where end-of-combat checks occur. This allows Draven to "see" the combat win before the board is cleared by the Decree's delayed effect.
• Attribution: As noted in the rules, the player who cast Imperial Decree gets credit for the kill, not the unit that dealt the damage.

Note: I could not find a card named "Sand Soldier." This ruling assumes a standard unit was used. If "Sand Soldier" has a specific ability that interacts with combat or triggers, please provide the exact card text for a more precise analysis.