Rostnsthal was once better known as Commodore Sthalmann Stalmannsyn, the commodore of Limsa Lominsa's Knights of the Barracuda. At the peak of his career, Sthalmann was responsible for ordering a self-serving and reckless operation that aimed to oust the current Admiral using the treasures of the fabled Seal Rock—a mission that succeeded in uncovering a ritual object for summoning primals.
But the operation soon went awry. With many of his subordinates slaughtered, and the artifact taken, Sthalmann's reputation was shattered beyond all repair. Stripped of his rank, he took up the name "Rostnsthal," (meaning "rusty steel" in the old Roegadyn tongue) and recruited his own crew of pirates in an ultimately unsuccessful bid to steal back the lost relic. The years following this failure were spent wallowing in cheap grog and misery, until an unexpected offer from distant Ishgard saw him accept the post of a marksmanship instructor.