Can I reveal a Hidden unit by playing a Reaction after the last item on a chain resolves but before the cleanup phase?

Ruling: You cannot play Reactions after a chain ends. Priority is only given to players at specific times, and when the last item on a chain resolves (after both players pass priority), the chain ends without giving priority again.

Sequence:

• Both players pass priority on the last chain item
• The last chain item resolves
• The chain is now empty
• Priority is NOT given to either player
• Cleanup phase occurs
• No opportunity to play Reactions exists between chain ending and cleanup

Nuances:

• Reactions can be played during a "closed state" (when a chain exists) if you have priority
• "Closed state" means a chain exists; "open state" means no chain exists
• Priority is only given at specific times: when a player plays an item, when both players pass on an empty chain in open state, when a chain item resolves if the chain isn't empty, and when the other player passes priority during a closed state
• A unit dying does not create a chain unless it has an ability like Deathknell that triggers upon death