Gaius Rhacelus is a Blood Angels Epistolary and is one of the few of the Chapter close to Mephiston, having been friends with the Chief Librarian when he was known as Calistarius.
Gaius was present when Hive Fleet Leviathan invaded the Baal System[1a], however he would not take part in most of the campaign against the Tyranids. Instead he was among the Librarians chosen to aid Mephiston in a Warp ritual that would prevent the Bloodthirster Ka'Bandha, from invading Baal as well[1b]. To his annoyance though, Mephiston also chose his current protege, the Lexicanum Lucius Antros, to be among their number for the ritual as well[1c]. Gaius had always felt, that Antros was too ambitious for power and thought it was unseemly how the Lexicanum constantly fussed over Mephiston. He had brought this to the Chief Librarian's attention, but was ignored and even asking Mephiston's only other friend, the Sanguinary Priest Albinus, to do the same ended in failure. However as they ventured into Baal's Cruor Mountains, where the ritual would take place, Gaius forcibly rebuked himself for doubting Anthros. If Mephiston judged the Lexicanum worthy enough to be his protege, then Gaius would accept Antros as well, and put aside any misgivings or doubts.[1b]
Later when the ritual began in a cave tunnel, a portal into Khorne's kingdom was opened and Mephiston struck at Ka'Bandha, in the hopes of ending the Daemon's threat before it could escape from the Warp. The Bloodthirster struck back however and then offered immortal life to any of the Librarians revoked their vows to the Emperor. Though none answered, Gaius could see that Anthros was falling under the Bloodthirster's sway and quickly told the Lexicanum not to fall for the Greater Daemon's false promises. This helped Anthros regain his senses, but the Lexicanum's weakness left a flaw in the ritual and Ka'Bandha's next attack caused the Librarians to collapse and the Warp portal was closed. Many of the Liberians who took part in the ritual were killed by the ordeal, but Gaius survived and quickly went to aid the despondent Mephiston. Though the injured Chief Librarian was shamed by their failure, he told the survivors that they would make amends by going to aid in the fight against Leviathan. Gaius however, informed Mephiston that Ka'Bandha's final attack had caused a cave in and he doubted they would free themselves in time to play a further role in the battle[1b]. He was proven correct, as when they escaped from the Cruor Mountains, Baal had been saved and the Hive Fleet had been defeated.[1d]
- 1: The Devastation of Baal (Novel)