If my opponent has a Sigarda, Host of Herons in play, and I play Armor of Thorns as though it had flash and then Donate it to my opponent, will the game lock up at end of turn due to an infinite loop of cleanup steps?
Sigarda, Host of Herons
Spells and abilities your opponents control can't cause you to sacrifice permanents.
Armor of Thorns
You may cast Armor of Thorns as though it had flash. If you cast it any time a sorcery couldn't have been cast, the controller of the permanent it becomes sacrifices it at the beginning of the next cleanup step.
514.3a At this point, the game checks to see if any state-based actions would be performed and/or any triggered abilities are waiting to be put onto the stack (including those that trigger “at the beginning of the next cleanup step”). If so, those state-based actions are performed, then those triggered abilities are put on the stack, then the active player gets priority. Players may cast spells and activate abilities. Once the stack is empty and all players pass in succession, another cleanup step begins.
I think that this question rests on who controls the ability that causes Armor of Thorns to be sacrificed. Is it my ability because I cast Armor of Thorns, or is it my opponent's ability because they control Armor of Thorns?