I was fighting Diablo, nightmare dificult, and my friend joined the game mid fight.
Before the fight started, diablo used his ''bone cage'' to lock my mercenary, i believe you`re meant to fight diablo alone if you`re doing single player.
But, because he joined the game after the fight was started, he could not enter the room, and left the game.
Right after he left, the mercenary that was locked in his "bone cage'' returned to me and helped me during the fight. I was using the templar.