In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, the creature you control from Macabre Mockery’s effect is exiled.

If you cast Macabre Mockery during a player’s end step, the creature will remain on the battlefield until the next turn’s end step. However, the +2/+0 and haste wear off during the cleanup step (which comes after the end step) of the turn in which Macabre Mockery resolved.