The Castellan was a cruiser used by the Galactic Republic

Galactic Republic Organization Galaxy-Wide Organizations

during the High Republic era

High Republic Era Event High Republic Era

.
While on a routine mission, it was severely damaged by a surprise attack from a fleet of Nihil ships, but managed an emergency jump to hyperspace to avoid destruction. It eventually returned to realspace in the Toshaal system

Toshaal System Galaxy Systems

and crashed on the surface of the planet Toshara

Toshara Planet

, but not before most of the crew abandoned ship in escape pods. The surviving crew and cargo were evacuated shortly after the disaster.
The wreckage of the ship remained on the planet in an inaccessible location, and over the years, scavenger

Scavenger Miscellaneous Roles

bands have salvaged it for valuables.