Main tip i can give for this is to play nice with the necromancer you meet on the lady vengeance in dallis's room. You'll meet him outside of stone garden graveyard once you reach act two and he'll ask you to go into one of the crypts and retrieve part of the sword, then the other part is in one the holding jails on blood moon island. Once you get both pieces give them to him and in the final act he'll give the repaired sword to you.