Do I need to declare all targets of Void Assault before a counter-spell is chosen to be played?

Yes, you must declare all targets (the friendly unit to be moved and the enemy unit to be moved) and all destinations (where each unit will be moved) when you play Void Assault, before any opponent has the opportunity to play a counter-spell.

Reasoning and Rules

• Targeting and Choices: According to the Patch Notes - 2025-10-24, move destinations are a required choice that must be made at the time of putting the spell on the chain. Furthermore, FAQ #6324 and FAQ #6318 clarify that you must declare all targets for a spell or ability when it is placed on the chain, before the ability resolves and before any reactions can be played.
• Execution: Void Assault contains two "Move" instructions. Per the Patch Notes - 2025-10-24, because move destinations are required choices, you must identify which friendly unit is moving, where it is moving, which enemy unit is moving, and where it is moving during the "Finalization" step of playing the card.
• Chain Timing: Once these choices are made and the card is finalized on the chain, the state becomes CLOSED. Only then do opponents receive priority to play a [REACTION] card, such as a counter-spell.

Summary of Sequence

1. Play Void Assault: You announce the card and pay the cost.
2. Declare Targets and Destinations: You choose the friendly unit, its destination, the enemy unit, and its destination.
3. Finalize: The card is placed on the chain as a pending item.
4. Priority/Reaction: Opponents now have the opportunity to play a [REACTION] (such as a counter-spell or other response) to the Void Assault currently on the chain.
5. Resolution: If the spell is not countered, it resolves, executing the move instructions in order.

Note: I could not fully verify this ruling against the retrieved rules, as the specific text and mechanics of the card 'Void Assault' were not present in the provided context.