What happens if I play Void Assault and my opponent plays Retreat on the target of Void Assault, does the Void resolve?

Yes, your Void Seeker still resolves.

According to Rule 359.3.e.5 and several Riftbound FAQs (such as FAQ #6100 and FAQ #4742), Riftbound spells never "fizzle" completely; you do as much as you can.

The sequence works as follows:

1. Chain: You play Void Seeker, targeting a unit on the battlefield. Your opponent responds by playing Retreat to return that unit to their hand.
2. Resolution: Because the chain resolves in reverse (LIFO - Last In, First Out), Retreat resolves first. The unit is returned to your opponent's hand.
3. Void Seeker Resolution: When Void Seeker attempts to resolve, it checks for its target. Since the unit is no longer on the battlefield, it is an illegal target.
   • The damage portion of the effect fails because the target is illegal (Rule 359.3.e.5).
   • Because the draw effect is a separate instruction (separated by a period), the "do as much as you can" rule applies. You proceed to draw 1 card.

Summary: The targeted unit takes no damage, but your spell successfully resolves its second instruction, and you draw 1 card.