A cross-game task where player-controlled characters, parties, or groups of characters complete for a reward.
A multiverse quest is a task that spawns across two or more games in a multiverse, which a player-controlled character, party, or group of characters may complete to earn a reward.
Here are a few interesting examples, from a game design perspective:
Another interesting quest type would be a multiverse main quest, where all games within the multiverse share the same meta-narrative, and a complex questline could spawn for tens, if not hundreds of gameplay hours across all games within the multiverse.