He was a companion to his fellow Librarian Ignis, who was known for fighting alone. Ctesias knew both of them and suspected that despite Ignis' nature, both he and Arcanakt understood each other. The two would later defend their Homeworld Prospero after it was attacked during the onset of the Horus Heresy and fought together in its capital Tizca's Collegia Azurine. There, they would kill a number of the invading Space Wolves side by side, before one of the Sons of Russ managed to strike Arcanakt down. Even after his death, though, Ignis remained fighting next to Arcanakt's body for a time. However near the end of the invasion, Ignis would eventually escape with the other surviving Thousand Sons to the world of Sortiarius.