Sothan by birth, he departed along with Captain Theodosios and the rest of the Company to investigate rumours of rebellion and uprising out on the Eastern Fringe. Unbeknown to the Fifth, this was the precursor to the Tyranid invasion of Hive Fleet Kraken.
When they encountered the first Hive Ship, Theodosios charged Cassios with leading a diversionary attack so that his own force could destroy it. This mission was a complete failure, and the entire Company was wiped out by Tyranid resistance. Unable to escape or regroup his forces, Cassios crawled into a small space inside the ship and activated his Sus-an Membrane.
Cassios remained alive in suspended animation for nearly three years aboard the Hive Ship, before being discovered and revived by Sergeant Tiresias and the 21st Salvation Team. Cassios was troubled by the loss of his men, and hungry for vengeance against the xenos. He could name each of his fallen comrades only from the sight of their remains, and felt that he owed them a debt in blood. He recounted his tale, then insisted that the 21st Salvation Team carry out his original mission to destroy the Hive Ship - Tiresias refused, citing Chapter Master Thorcyra's new conservationist policies.
Neophyte Pasan persuaded the rest of the team to lie to their sergeant about the presence of more survivors, but instead intended to let Cassios re-enter the Hive Ship and destroy it - the commander's intention was to undertake a suicide mission, using a brace of explosive mining charges.
The entire 21st Salvation Team was wiped out in the course of the mission, which ultimately failed despite the destruction they unleashed in the heart of the Hive Ship, killing countless Tyranid creatures. Severely wounded, Tiresias used a Reductor to extract both Cassios's and his own gene-seed before losing consciousness. Tiresias's body was later recovered by Sergeant Quintos and the 121st Salvation Team.