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:
Loop Quest: An item collected in Game A unlocks a chest in game B; an item collected in the chest in Game B unlocks a dungeon door in Game C; an item collected in Game C’s dungeon leads to a hidden treasure in Game A.
Collection Quest: Items collected in Games A and B unlock a chest in Game C.
Set Quest: Items collected in Games A, B, and C are part of the same set (e.g., a set of armor).
Non-Linear Quest: A complex questline that leads a player across multiple games in a random fashion (e.g., Game A, B, C, B, C, B, A, B).
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.