If I attack an empty battlefield with Immortal Phoenix, can my opponent kill it with Void Seeker or does it have assault?

If you move your Immortal Phoenix to an empty battlefield, it does not gain the +2 Might from Assault.

According to Rule 733.1.c, Assault is functionally short for "While I am an attacker, I have +X [M]." Rule 733.1.d clarifies that "Being an attacker means the Unit has gained the Attacker designation during Combat."

When you move a unit to an empty battlefield, you are not entering combat (as there is no opponent to defend). Because there is no combat, your unit does not gain the "Attacker" designation, and therefore the Assault ability does not activate.

Interaction with Void Seeker

Since the Immortal Phoenix remains at its base Might of 3:

1. Void Seeker is an [Action] spell that can be played during a showdown or on your turn.
2. If you move to an empty battlefield, you have not started a combat, but you have moved to a location where your opponent can interact with you if they have priority.
3. Because the Phoenix only has 3 Might, it is a valid target for Void Seeker (which deals 4 damage).

Summary: Your Immortal Phoenix will have 3 Might, not 5, and can be killed by Void Seeker.