An "attacking creature" is one that has been declared as an attacker this combat, or one that was put onto the battlefield attacking this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game, or the planeswalker it was attacking has left combat.

The targeted creature doesn't have to be attacking you. It can be attacking a planeswalker, or (in a multiplayer game) it can be attacking another player.

At the beginning of the next end step, the affected creature is returned to the battlefield even if Mystifying Maze is no longer on the battlefield by then.