Tiber is a peasant in Baldur's Gate who is encountered standing in the shade of a large tree on the other side of the creek from the spot where the Gorion's Ward and party enter the Cloakwood Nest area.
Tiber and his brother Chelak decided to go into the Cloakwood to clear the woods of a spider infestation. His brother had found the enchanted sword Spider's Bane and they thought they'd become famous for performing the deed. He asks you to look for his brother, who went into the woods a week ago and hasn't returned. If you agree to search for Chelak, the side quest Spiders in the Cloakwood is added to your Journal in the Quests section. Tiber has an exceptionally timid and cowardly demeanor, so it is hardly surprising that his brother left to explore the Cloakwood alone.