From Lexicanum

Captain Ardias of the Ultramarines Third Company, led the Third Company against the Tau but later negotiated a cease fire with them when Governor Severus aligned himself with the daemon Tarkh'ax and the Word Bearers.

Ardias forced an override in the communications of a Tau Fire Warrior named La'Kais and guided him against the Word Bearers and subsequently helped Kais by holding of Tarkh'ax until Kais destroyed the daemon.

Sadly and ironically Captain Ardias was later seriously injured battling the Tau in the Joran retaliation during the Damocles Crusade in 745.M41.

Sources

• Fire Warrior (Novel) by Simon Spurrier