- This sidequest requires The Keyring Mod.
Head to Lord British's castle in Britain and talk to the sage standing beside the drawbridge. He introduces himself as Zauriel, and goes on to explain how he has been waiting for you to arrive and help him rescue his daughter Laurianna.
Agree, and he will explain that in order to neutralise the effects of the necklace you need a chunk of Blackrock. To make the potion you need a second chunk of Blackrock as well as a vial of Silver Serpent venom. You can acquire the Blackrock from Rudyom in Cove, or from the Britannian Mining Company's mines in Minoc. You can acquire the Venom from Minoc, or from other places if you nose around.
Once you have the ingredients, return to Zauriel and choose Make potion. He will then teleport your party to his lab, make the potion, and tell you where his daughter is being held. She is held in the northwestern corner of an island north of Skara Brae. After this revelation he will teleport you back. He informs you that Britain is too crowded for him, so he relocates to 45S 38W, not far from where his daughter is being held.
As Zauriel mentioned in passing, this potion is yet another way to kill Lord British. You can also try it on Batlin if you wish. Remember to save ahead of time in both cases, as you can't get a replacement.
You'll find the island at around 3S 69W.