Case of the Burning Masks

"Solved — [Triggered ability]" means "[Triggered ability]. This ability triggers only if this Case is solved." Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]."

A permanent that deals damage multiple times without changing zones still counts as only one source. For example, if you attack with Incinerator of the Guilty and deal combat damage with it, use its triggered ability to deal damage, and then use Hard-Hitting Question to have it deal damage to another creature, it's still just one source even though it dealt damage three times during the turn.

You pay all costs and follow all normal timing rules for the card played from exile with Case of the Burning Masks's last ability. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.