At some point, he was transferred to the Deathwatch where he continued to serve under the Inquisition's Ordo Xenos kill-teams and held the rank of Captain. His long campaign had meant that his name inspired terror within a number of Eldar and Ork warlords. At some point, he acquired an Eldar blade from a defeated foe from the Biel-Tan Craftworld. Captain Octavius frequently fell under the service of Inquisitor Agustius and at some point was assigned on a mission where he was to meet with the Inquisitor in the Segmentum Obscurus. However, during transit, his team discovered the Herodian IV warp gate being blocked and instead he was recruited to serve under Inquisitor Lord Brutius Parthon who dispatched his agent Kalypsia to direct Captain Octavius's team. They travelled to the Herodian system where they joined the Imperial Navy forces there who were struggling to combat a rogue splinter of Tyranids that had mysteriously arrived and attacked Herodian IV. Despite the presence of the Mantis Warriors, the Imperial forces were incapable of defeating the Tyranid forces and if the planet fell than the link to the Segmentum Obscuras would fall as well due to the importance of the warp gate. Despite the insistence of Captain Kastor who suggested that an Exterminatus be conducted to purge the planet of the Tyranid infestation, Inquisitor Kalypsia instead ordered that a mission be conducted to target the ruling Hive Tyrant within the swarm in the belief that its destruction would cripple the Hive Fleet.
Initially, the Inquisitor ordered for a drop mission by Thunderhawk be conducted at a specific location on the planet. However, Captain Octavius refused the order as it was in the middle of the Tyranid swarm and thus they risked destruction. Instead, he directed his team to another site but during the flight, their gunship was attacked by Gargoyles which saw the machines destruction. Though this was the case, Octavius and his men managed to land safely alongside Kalypsia. However, there they were ambushed by Tyranids who seemingly knew of their landing position and a Lictor was responsible for killing Sergeant Grevius. Two further deaths fell upon the kill-team when Tyrian Kulac of the Space Wolves and a member of the Crimson Fists both died in battle against the Tyranids. It was only the arrival of Librarian Shaidan of the Mantis Warriors and his kin did the Deathwatch forces survive. Shaidan's psychic powers were used to help that of the kill-team's own Librarian, Ashok in holding back the Tyranids until they could retreat. After returning to orbit, Captain Octavius concluded that the Tyranids seemingly were fully aware of Codex Astartes tactics and the strategies of the Deathwatch but Inquisitor Kalypsia dismissed his views as such concluded that their Xenos foes were mere animals. During his time in orbit, Quirion Octavius became somewhat suspicious of Kalpysia after he came to believe that she murdered an Adeptus Mechanicus survivor from the planet but despite his views, he continued to serve. In addition, in order to bolster his forces after their loss on the surface, he added into his kill-team Shaidan, Soron and Ruinus though they were not considered Deathwatch.
Their next attempt was done so under Inquisitor Kalypsia's direct orders and insisted on a surgical strike of the coordinates she had initially desired. The Captain of the Deathwatch relented and made use of a Drop pod to bring his forces to bear at the location. Kalypsia later directed the team into an unknown Imperial structure which was later discovered to be a research facility and inside they discovered the body of an unknown Inquisitor. It was then that the Mantis Warrior Shaidan revealed that he and his Chapter had originally uncovered the base and learnt that the machine to which the Inquisitor was in was a weapon. They activated it and initially it turned the Tyranid tide but the xenos eventually regrouped and overran their position. The Inquisitor sought to recover the device but Captain Octavius was more concerned with achieving his objective namely the destruction of the Hive Tyrant - though was unaware of the fact that the unknown machine was actually the real objective of Inquisitor Kalypsia. During the recovery operation of the machine, the Hive Tyrant appeared and both Chaplain Broec and Shaidan died whilst in the process of slaying the beast. In addition, the Mantis Warriors within the team such as Soron and Ruinus were also killed in the operation. Eventually, Octavius and his surviving kill-team managed to escape in a Thunderhawk but the unknown machine bonded with Kalypsia and turned her into a powerful psychic as well as possessed her to the point that she turned her powers against the Deathwatch team. She was ultimately jettisoned into space and the chair she was forcibly bonded to was remotely detonated and destroyed.
Returning to the fleet, Captain Octavius informed Captain Audin of the Mantis Warriors of the death of his comrades. He also ensured that they be known as members of the Deathwatch with their promotion being a post-humous award due to their bravery. Captain Octavius later learnt that the Tyranids eventually became disorganized and defeated. It was then, Inquisitor Lord Parthon arrived and asked for a report. Captain Octavius did not reveal his findings of the Grendal-machine and simply stated that the Hive Tyrant was destroyed with Kalypsia "falling" in combat. When the Captain asked about the possibility of returning to Inquisitor Agustius's service, Lord Parthon simply stated that Agustius had died. He later said that he would be taking charge of Captain Octavius's kill-team for a mission after which he ordered an Exterminatus against Herodian IV in order to wipe out the Tyranid forces.