Priest of Ilmater (Baldur's Gate)

This Priest of Ilmater is found in the Temple of Ilmater which is located in the western section of Northeast Baldur's Gate. He only appears during Chapter Seven. He'll offer to show you his secret passage to the Undercellar either through the initial dialogue, if you choose the right dialogue path, or if you speak to him again after the first dialogue attempt has ended.
This is a one-way travel action and there is no way to get back to the temple unless you exit back to the outside. The priest doesn't actually accompany you when you exit into the Underceller and he can still be found in the temple if you return to it.
The priest doesn't offer any services for the temple.