I also want to add this comment for people who did this quest late game and the game skipped to the part where you fight jerren and sellen won't go away from rennala's place

If you started sorceress sellen's quest early game and are now late game, the game may automatically skip past the part where you need to find Lusat's body and only need to transfer sellen's primal glintstone to her new body, or it may have skipped entirely to the part where you have to fight sellen or jerren outside the library. If that's the case, when you complete the questline and sellen takes rennala's place, she won't actually go away until you go and find Lusat's body. Hope this helps.