Talk:Lionhead Spider Cultists' lair/@comment-207.6.82.252-20151220111958/@comment-2604:2D80:402D:891C:12C:226:4AE8:F6A9-20171011115420

It is an unofficial quest, gained by finding the dead knight (while or after hunting the cockatrice). Suprrised this article doesn't link it since it's on the same site, but a Google search found it:

http://witcher.wikia.com/wiki/The_Cultists%27_Crypt

Hope this helps.