Does Arcane Shift require a valid enemy unit target to deal 3 damage to in order to be played, or can it be played just to blink a friendly unit when no enemy units are at a battlefield?

Ruling: Arcane Shift requires valid targets for both parts of the effect (a friendly unit to blink AND an enemy unit to deal 3 damage to) in order to be played on the chain. You cannot play it if there is no enemy unit at a battlefield.

Nuances:

• Arcane Shift is not a reflexive trigger because it does not contain the phrase "do this" after "then"
• If a target becomes invalid after the spell is on the chain (e.g., killed by Hidden Blade in response), there is no fizzle in Riftbound - you simply do as much as you can (blink the friendly unit but don't deal 3 damage)
• This follows the same principle as other spells like Stupify, where you cannot play them just to get partial effects if all targets aren't valid