Can I Mystic Reversal a hidden card and choose a unit at another battlefield?

No, you cannot use Mystic Reversal to change the targeting of a hidden card to a different battlefield.

Here is the breakdown of why this interaction is restricted:

1. The Targeting Restriction on Hidden Cards

According to the established rules and FAQs for Riftbound, hidden cards are strictly bound to the battlefield where they were hidden.

• Rule: A card played from hidden at one battlefield must target units and place tokens only at that same battlefield (FAQ 5706).
• Exception: The only known exception to this rule is Tideturner, which has specific errata allowing it to target units at other locations because it cannot fulfill its targeting requirements at its own location (FAQ 6517, 7602).

2. How Mystic Reversal Interacts with Hidden Cards

When you use Mystic Reversal to take control of a spell (or a card being played as a spell, such as a hidden card being flipped), you are allowed to make new choices for it. However, you are still bound by the legal requirements of that card.

• Legal Targets: Because the card was hidden at a specific battlefield, its "legal" targeting area is restricted to that battlefield.
• The Limitation: Even though you are now making the choices, you cannot choose a target at a different battlefield because that target would be illegal for that specific card. You must select a legal target at the battlefield where the card was hidden.

Summary

If you use Mystic Reversal on a hidden card, you may change the target, but you must choose a target that is at the same battlefield where the card was hidden. You cannot use this interaction to bypass the fundamental restriction that hidden cards are tied to their specific battlefield.