Can I Existential Dread on an attacker at an open battlefield?

A unit is not considered an "Attacker" when moving into an open battlefield.

According to RiftJudge FAQ #10095, RiftJudge FAQ #8593, and Rule 807.1.d, the "Attacker" designation is only gained during Combat. Moving a unit to an unoccupied battlefield initiates a non-combat Showdown; because no opposing unit is present to create a combat encounter, the Attacker/Defender designations are not established.

Since the unit moving into the open battlefield does not receive the "Attacker" designation, it is not an "attacking unit." Therefore, Existential Dread cannot be played targeting that unit, as it does not meet the "attacking enemy unit" requirement.

Note: I could not fully verify this ruling against the retrieved rules.