The Golden Sarcophagus is a large, spherical structure on Baal that is slightly translucent with golden light emanating from it. This sarcophagus is where the Great Angel, Sanguinius of the Blood Angels, was laid to rest after his death at the hands of the Arch-Traitor Horus during the Horus Heresy.
During the attack on the Blood Angels' fortress-monastery by the rabid mutants known as the Bloodfiends, the Blood Angels and their Successor Chapters such as the Flesh Tearers, Blood Drinkers, Blood Swords, and the Angels Sanguine, stood together to defend the Holy Sepulchre, which housed the Golden Sarcophagus.
As the fight became dire, a single, highly-developed Bloodfiend made its way through the ranks of Space Marines, even getting past Chapter Masters Dante and Gabriel Seth on its way to the sarcophagus. After throwing aside the last line of defense, including Chaplain Argastes and Chief Librarian Mephiston, the "Alpha" Bloodfiend faced a single Blood Angel, Sergeant Rafen in single combat. The Bloodfiend seemed unstoppable even after Rafen stabbed his combat blade into its heart, but Mephiston threw his force sword, Vitarus, to him. Even though Rafen was unable to conduct psychic powers into the blade, it was still a formidable weapon, with which Rafen pierced the Alpha's secondary heart, bringing it down within a hand's reach of the Sarcophagus.