If I void assault a unit to a battlefield but they react on that battlefield to the void assault with an Emperor's Divide, what happens?
When you play Void Assault and your opponent reacts with Emperor's Divide, the sequence of events depends on the specific targeting of each card and the nature of the instructions.
1. Targeting and the Chain
- Void Assault targets one friendly unit and one enemy unit. Per FAQ #8796, choosing these units creates a lasting targeting relationship between the spell and those units.
- Emperor's Divide is played in reaction, placing it on top of the chain.
2. Resolution (LIFO)
- Emperor's Divide Resolves: It moves the friendly units of the player who cast it from the battlefield to their owner's base.
- Void Assault Resolves: It attempts to execute its instructions in order ("Move a friendly unit, then move an enemy unit").
- First Instruction: It moves the friendly unit targeted by Void Assault. Since Emperor's Divide (played by the opponent) only affects their own units, your targeted friendly unit remains on the battlefield. Void Assault successfully moves it to the designated battlefield.
- Second Instruction: It moves the targeted enemy unit. If that unit was moved to the base by Emperor's Divide, the targeting relationship is maintained because the unit remains on the board (a valid game object). Void Assault will move that enemy unit from the base to the battlefield.
Conclusion
Void Assault does not fizzle or fail. It continues to act upon its targets as they exist at the time of resolution, effectively moving them to the destination battlefield regardless of whether they were displaced by Emperor's Divide.
Note: I could not fully verify this ruling against the retrieved rules.