Marchosias was a British man and Viktor's servant, until his master decided to remove his head and heart from his body as a form of punishment. Viktor then attached his head onto the body of a small dog and locked him in a shed.
Marchosias first encountered Josh in the mansion's garden, where he called him out from the shed begging for help to find the shed's key. Once unlocked, Marchosias warned Josh not to be alarmed by the current state of his "body." Marchosias promised to help Josh in exchange for his assistance to regain his stolen heart, which he needs to animate his real body. After Josh retrieved his heart from one the creatures that stole it, Marchosias proceeded to reattach himself and as promised, rewarded him with a lever that would unlock the tower containing a "surprise" for him. Marchosias wished him luck and the two parted ways.
Marchosias later returned, knocking Josh out from behind after he had defeated Viktor at the ritual altar. Marchosias later carried Josh, and the lifeless bodies of Viktor and Ernst to the same laboratory he used to regain his body. He explained to Josh that although he was angry at Viktor for what he did to him, he felt much more magnanimous since he regained his body. He also remarked that Viktor is a great mind pushing the limits of mind and body, and even though he admitted that Viktor's work was unethical and insane, he proclaimed the mad doctor as a genius. Marchosias proceeded to extract a parasites, containing Viktor's mind from the old body and passes it into Josh's body while Josh's mind was planted inside Ernst's lifeless body.
Marchosias later encountered Josh at the same chamber he had obtained Azoth, pitting him against the altered Jane. Upon defeat, she turns against Marchosias and immolates herself along with him. Marchosias managed to survive Jane's immolation, though his entire body was badly burned in the process and he had to cover his body with bandages. He later boards the same train Josh and Kate boarded, with Josh unaware of his identity and pass by each other. Marchosias brought along with him phials of Viktor's remaining parasites hidden within his briefcase.