I forgot to mention: You do not need to kill him. He is kind of optional. He would award you with enough rift energy to switch back to the first reality and give you a damage boost but you can get the energy from the other mobs/chests. The final boss will be in the first reality but you can not go directly to him as there some objects blocking the access. So the path is basically: collect rift energy in the first reality. Go to the barrels that block your way (there will be a voice over). Switch realities, collect rift energy go to the next blocking objects -> switch realities and defeat the evil and reforge the sigil.