He was the great-grandfather of Crescius Caerellius of Raven Rock. Gratian was grievously wounded by Draugr within Raven Rock Mine, after he was sent by the East Empire Company to explore it. Gratian managed to recover the Bloodskal Blade, but couldn't figure out the puzzle needed to advance further into Bloodskal Barrow. He died from his injuries, and the mine was shut down. The East Empire Company claimed that he was killed in a cave-in so that they wouldn't have to send any more explorers.
The remains of Gratian Caerellius can be found inside Raven Rock Mine, at the top of the stairs, just ahead of the remains of his assistant, Millius. On his corpse his journal, and the Bloodskal Blade can be found. Delivering the journal to Crescius Caerellius gives him proof needed to reopen the mine, bringing Raven Rock back to a relative state of prosperity.