During Act 2, Yeet the flask that has the beholder/spectator and let it turn into an undead. And control it for as long as possible.
Unfortunately, I gave the chest to Zarys for Harold and then forgot to pickpocket it back off her before I fast travelled so it disappeared forevermore.
Ironically, I forgot to pickpocket it off her because I went to buy the gloves of thievery from Brem first. After buying them and sorting equipment I immediately fast travelled and slapped myself on the forehead.