If an opponent plays hidden blade from hand, and i react with a hidden tidehunter, does it save my unit being targetted?

To determine the outcome of this interaction, we must look at how the chain resolves and how targeting works in Riftbound.

The Chain Sequence

1. Opponent plays Hidden Blade: This is a spell that targets a unit at a battlefield. It goes on the chain. You are the current defender, and the state is now CLOSED (a chain exists).
2. You react with Tideturner: Since Tideturner has the Hidden keyword, it gains [Reaction] when played from facedown, allowing you to play it during a CLOSED state. It goes on top of the chain.
3. Chain Resolution (LIFO - Last-In, First-Out):
   • Tideturner's ability resolves first: When Tideturner enters play, its triggered ability goes on the chain. It allows you to move Tideturner to the location of a friendly unit you control elsewhere and move that unit to Tideturner's original location (swapping their positions).
   • Hidden Blade resolves last: After the swap is complete, Hidden Blade attempts to resolve.

Does this save your unit?

Yes, it can.

According to Rule 359.3.e.9, when a spell targets a unit at a battlefield, if that unit is moved to a different battlefield (or its base) before the spell resolves, the spell is still targeting that unit. However, Hidden Blade specifies "Kill a unit at a battlefield."

• If your unit was at a battlefield and you swap it with Tideturner (who is also at a battlefield), your unit is now at a new location. As long as it is still at a battlefield, it remains a legal target for Hidden Blade, and the spell will successfully kill it.
• However, if your unit was at a battlefield and you swap it with Tideturner (who was at a battlefield) and the move results in your original unit ending up in your base (a non-battlefield zone), the target is no longer at a battlefield. Hidden Blade's instruction to "Kill a unit at a battlefield" cannot be followed for that target, and it will fail to kill your unit (see Rule 359.3.e.9 and Rule 359.3.e.10).

Summary: Swapping your targeted unit to a battlefield location saves it only if the spell's specific targeting requirements (in this case, "at a battlefield") are no longer met when the spell attempts to resolve. If you swap it to a non-battlefield zone (like your base), it is safe.