If Riptide Rex's target moves to base, dies, or returns to hand during reaction, does it still deal 6 damage or can you choose a new target?

Ruling: Riptide Rex will deal 0 damage if the target is no longer at a battlefield when the effect resolves. You cannot choose a new target.

Sequence:

• Targets are declared when the effect is added to the chain
• Target legality is checked again when the effect resolves
• If the target is not at a battlefield upon resolution, the effect deals 0 damage

Nuances:

• Targets cannot change once declared
• This applies whether the unit moved to base, died, or returned to hand during reaction