I swear I've seen cards that require returning opponents exiled cards to the graveyard as a condition to cast but can't find them with any search combinations. Was I dreaming?
You are probably thinking of the processors (e.g. Blight Herder) from the Battle for Zendikar and Oath of the Gatewatch sets. (Though they mostly have abilities that do extra stuff if you return cards, not a requirement for actually casting them.) As well as some instants/sorceries from the same sets (e.g. Processor Assault).
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)