Martellus is a Techmarine who serves aboard the Blood Ravens Strike Cruiser Armageddon. He assisted the Force Commander during Aurelian Crusade in Dawn of War II. Martellus supported the Blood Ravens by providing them with intel regarding enemies, later leading a large group of survivors from the Armageddon planetside as the Tyranid Hive Fleet overwhelmed and subsequently annihilated the Armageddon in orbit. He piloted a Thunderhawk-2 in support of the Force Commander but did not make it to the battleground and was perceived to be lost.1
A year later during the second Aurelian crusade, Martellus was discovered sending a distress signal from an Astronomic Array not far from the site of the final battle with the Tyranids. He claimed to have been battling his way there, wounded and with minimal ammunition for the majority of the past year. He once more helped the Force Commander by providing critical data on the sub-system and helping to identify the traitor in the midst of the Blood Ravens.
In the Game
In-game, Martellus is potentially the traitor that aids the Black Legion in sabotaging and deceiving the Blood Ravens. If the player completes the majority of the game with all squads "Pure", Martellus is the only character in the game that can be identified as the traitor. He then attacks the player on the surface of Aurelia with a heavily modified Chaos Predator Tank.2
According to information about Dawn of War II: Retribution , events of which take place 10 years after Chaos Rising, canonically Martellus wasn't a traitor.3
1 Dawn of War II - Final mission