If Void Assault moves my unit and an enemy unit into an open battlefield on my turn, who is the defender?

When you play Void Assault and move your friendly unit and an enemy unit into an open (unoccupied) battlefield, the enemy unit becomes the Defender.

Why?

According to the combat rules and the mechanics of Contested status:

1. Contested Status: When you move units to an open battlefield, that battlefield becomes Contested. As the player who moved their unit there first, you are the player who applied the Contested status to the battlefield.
2. Attacker Designation: Per the mechanics described in RiftJudge FAQ #9407 and Rule 445, the player who applies the Contested status is designated as the Attacker.
3. Defender Designation: Because units from both players are now present at the battlefield, a Combat is opened (Rule 447). The player controlling the opposing unit present at the battlefield is designated as the Defender.

Summary

• Attacker: You (the player who controlled the units that applied the Contested status).
• Defender: Your opponent (the controller of the enemy unit).

This interpretation is consistent with how "Contested" status dictates combat roles in Riftbound.