Girduin was one of the students who accompanied Atmah on her expedition to Labyrinthian.
He is the first to mention that since the place has a "history" of power, there's likely something of magical property to be found within. He also chides Atmah that she's "the Arch-Mage's favorite" student when she says that she wants to get in and out as quickly as possible.
Girduin is the first of the mages to die when the group was ambushed by a Skeletal Dragon and numerous Skeletons. In the next room, Elvali screams that they can't leave Girduin behind, while Hafnar Ice-Fist shouts, "We barely made it out alive, and you want to go back in?" and one of the mages states that there "Isn't enough of him left" to go back for, suggesting that he was mauled to death.
Girduin's ghost is found in Labyrinthian during the quest, where one can hear him conversing with his fellow students.