The Key of the Caretaker is a essential tool in Necromancy to create spells. It isn't a real key, but a metal pole with a small fake skull on the top. The Key is used on a closed pouch with the reagents and the right Words of Power to create the desired spell talisman.
The Avatar received the Key of the Caretaker from Vividos in Ultima VIII, after entering the order of the Necromancers and bringing him the reagents Executioner's Hood and Dead Man's Elbows. With the key in the Avatar's possession, the creation of spells became possible.