When a unit with a hidden card is moved from a battlefield using Charm, when can the opponent cast the hidden card, and what happens if they miss that window?

Ruling: Hidden cards have the Reaction keyword, so the opponent can flip and cast the hidden card in response to Charm being cast. If they miss that window, the hidden card is trashed during the cleanup step after Charm resolves and before any move triggers go on a new chain.

Sequence:

• Charm is cast and target is declared
• Opponent can respond by casting the hidden card (using its Reaction keyword)
• If not cast, Charm resolves
• Chain closes and cleanup step occurs
• Hidden card is trashed during cleanup
• Move triggers (if any) go on a new chain

Nuances:

• The opponent must cast the hidden card in response to Charm or lose it, as there is no window after Charm resolves
• Move triggers happen too late - the hidden card is already trashed in the cleanup step before move triggers can be responded to