You need to use the 3 talismans in the correct order at a statue in the derelict tunnels where you found them, in the central region. There was a book there mentioning a statue, but not the location. You get the talisman of the north after that (and other random stuff in the chest that appears), which IIRC has +2 Wisdom, 3 enchantment slots and a couple other random bonuses.
