This is a fun and amusing quest. It is begun by finding Grizelda the Hag, skulking around in her hovel in the Brittanian countryside. Talking to her, she makes you an offer, in exchange for help she will make it worth your while...
The witch is somewhat demanding. She will have you running over the land, to fetch her this or fetch her that. There is a long list of possible things she could ask you for, some straightforward and others a bit more ambiguous. If you wish to spoil the fun of figuring it out for yourself, you can consult this page. Afterwards you must find an imp for her. You are provided with a map which may or may not be easy to read. If you need help finding the location, consult this page.
Upon completing the quest, the witch will give you a few rare items, some reagents, a magic item, around 2000 gold, and a treasure map.