In a multiplayer game, what happens if I use a card like Threaten to temporarily gain control of a permanent, then give control of that permanent to a different opponent using something like Zedruu the Greathearted? Does control of the permanent return to its owner at the end of the turn? Or does the second opponent maintain control of it indefinitely?
The second opponent will retain control of the permanent indefinitely, since the Zedruu effect giving that player control of the permanent has no duration (unlike with Threaten's effect), so it lasts for the rest of the game (C.R. 611.2a).
See also this thread and this thread.