If I move units to an open battlefield, then cast Moonfall to move an enemy unit to my battlefield, who is considered the attacker?

To determine who is the attacker, we must look at which player applied the Contested status to the battlefield.

According to RiftJudge FAQ #8263 and Rule 445, the attacker is the controller of the units that applied the contested status to the battlefield.

Sequence of Events

1. Initial Move: You move your units to an open (uncontrolled/unoccupied) battlefield. By doing so, you apply the Contested status to that battlefield. You are now the player who applied the contested status.
2. Casting Moonfall: You cast Moonfall to move an enemy unit to that same battlefield.
3. Combat Designation: Because you were the player who initially applied the contested status to the battlefield, you remain the attacker. The enemy unit moved by Moonfall becomes the defender.

Summary

• Attacker: You (the player who first moved to the battlefield and applied the Contested status).
• Defender: Your opponent (the player who controls the unit moved by Moonfall).

This is based on the principle that the attacker is determined by the controller of the units that first applied the contested status, not by which player played the spell that moved units into the battlefield.