Help me pls, im stuck in act 5

I have a problem in act 5 of the boss on the first map, I don't have a quest item that I should give in order to move on, because I don't get it, I can't unlock the door and move on in the story. I'm asking for help, this is my second character but this is the first time this situation has happened to me. Does anyone know a solution to this problem? I would be very grateful