Tiber is a peasant in Baldur's Gate. Tiber has an exceptionally timid and cowardly demeanor, so it is hardly surprising that his brother Chelak left to explore the Cloakwood alone.
He can be found just north of the spot where the protagonist enters Cloakwood Nest. He asks you to look for his brother Chelak, who went into the woods a week ago with the enchanted sword Spider's Bane. If you agree to search for Chelak, the side quest Spiders in the Cloakwood is added to your journal in the Quests section.