At some point, Seb enlisted into the military and joined a FEDRA unit led by Avery with squad members Irina, Mali, Victor, and an unnamed sixth person. Their squad was stationed at the Seattle quarantine zone. At some point, FEDRA ordered Seb's unit to take out an infected nest in Hotel Blacray by destroying the building's support with explosives. However, the detonators were faulty and the squad was trapped in the hotel by a group of stalkers, ultimately resulting in their deaths.
Events of The Last of Us Part II
Abby Anderson came across the corpses of Seb and his squad while searching Hotel Blacray for a gas mask for Lev. She can find the FEDRA orders and the FEDRA final note artifacts chronicling their fate.