Martellus served with the Force Commander throughout the Tyranid invasion of sub-sector Aurelia. During the climactic battle over Typhon Primaris, Martellus was aboard a Thunderhawk gunship supporting the Blood Ravens ground forces. The Tyranid fleet eventually overwhelmed the gunship, forcing it to crash-land, and Martellus was assumed to be lost to the Chapter.
Unknown to the Chapter, Martellus had survived the crash, rendered immobile and without means of communication. Over the course of several months Martellus was able to repair himself enough to restore mobility. However, he was stranded alone in the jungles of Typhon, still infested with feral Tyranids.
Eventually Martellus managed to make his way to the Astronomic Array the Chapter had previously used to scan the Hive Fleet – a tendril of Leviathan – and sent a message that was later picked up by the 3rd and 4th Company forces guarding the sub-sector. A small deployment of 4th Company ground troops extracted Martellus along with data from the Astronomic Array.
If none of player's squads are corrupted then Martellus will be revealed as the traitor leaking Chapter secrets to the Black Legion. After the crash of his Thunderhawk, Martellus had felt abandoned and betrayed, as the Chapter had assumed he did not survive the crash. Eliphas took advantage of the situation and helped Martellus, persuading him to join the Black Legion to take revenge on his brothers. Martellus later managed to expose himself as the traitor and escape to planet Aurelia at the same time, and the Force Commander led a mission to execute the traitorous Techmarine. Martellus appeared in a heavily modified and extremely powerful Chaos Predator. Martellus could be considered the hardest Traitor to get, as it requires all squads to be completely pure, meaning all objectives that cause Corruption must be avoided and those that give Purity must be completed.
Martellus appears in Retribution as a hero under the player's command. He accompanies Diomedes throughout the campaign, and is particularly important due to his skills as a Techmarine, which allowed the Blood Ravens to use the Teleportarium system in their covert mission.