At Mare Imbrium a host of thousands of Guardians came united to retake the Moon, and were mercilessly slaughtered by the seemingly invincible Crota and his army of Knights.1
Following this great disaster Eriana-3 and Eris Morn sought to find another way to kill Crota. They consulted Toland the Shattered, the "exiled master of Hive arcana", who told them that Crota existed in another reality, one where he could be defeated.
They were all believed to have perished in the Hellmouth. After many years, Eris Morn returned without her Ghost, having somehow survived amongst the Hive, with tales of her Fireteam's failure and the demise of her comrades at the hands of Crota's disciples.
Toland was also believed to have perished in the Hellmouth, but since the arrival of Oryx we have received a number of messages from Toland.
The other Fireteam members are presumed dead.