According to his own narration, Marius used to live in the Rogue Monastery, but what his position there was is never revealed. He had fled when Diablo's hellspawn overwhelmed the citadel, finding refuge in a small hostel along with other outcastes and fighting nightmares of the attack. But Diablo, in the guise of the Dark Wanderer, stopped at the same hostel, and the place was quickly destroyed by the demons that his presence called up. When he left, Marius was compelled to follow, and chronicled the journey from the forests of Khanduras to the gates of the Burning Hells.
Marius did not know that he was travelling with the Dark Lord of Terror himself. When their caravan reached the deserts of Aranoch, he started getting visions of Baal's capture in his Soulstone. But, he had no idea of what he saw and the only clue he got was from the Wanderer that they were here to free his brother. When they reached Tal Rasha's tomb, he sensed that the Wanderer was not as human as he thought. On reaching Tal Rasha's Chamber, he misunderstood the Archangel Tyrael's presence there and fell under Baal's pleas to release him. He was instrumental in the release of Baal from his prison by removing his Soulstone from his chest. Tyrael caught hold of him and he was given the charge of going to the Burning Hells and destroying the soulstone that held the demon's essence at the Hellforge.
Unfortunately, he could not bring himself to enter the portal into hell, because he ended up seeing the Three Prime Evils together summoning the very portal that he intended to go through. He also witnessed the Wanderer's transition into Diablo's physical manifestation. Completely horrified and utterly confused at the horrors that he saw, he ended up in an asylum. His experiences had quite clearly driven him insane.
The cinematic cut scenes help depict Marius' story. Marius thinks he is telling the story to Tyrael but he is, in actuality, conversing with Baal who is pretending to be Tyrael. He tells his story after being asked by Baal:
- "Now tell me Marius, how was it not your fault?" (refering to Marius' actions since last seeing Tyrael).
- "Tyrael... Yes... Yes, now I recognize you. I should've known you would travel in disguise. They're... They're always watching."(upon gazing at the cloaked figure)
- "Oh, forgive me, Tyrael, please. It... wasn't my fault."
- "The Wanderer... Yes, it was... the Wanderer."
- "My days at the Rogue Citadel seemed so long ago... I sought refuge from my memories in the company of other outcasts, high in the mountains, past the Eastern gate."
- "Oh, I'd fought sleep for days at a time... For when I dreamt, the memories would return. Memories of the Monastery, and the Evil which had claimed it! Dreams, memories... I couldn't tell the difference anymore."
- "Had the Evil from my dreams followed me at my heels? How had he found me here? How could this broken shell of a man, barely able to carry the weight of his own sword, be the Burning Terror which drove me to hide here?"
- "He seemed to have Demons of his own, that he struggled to contain... and he was losing."
- "As I watched... I... became convinced that I was truly mad. The Terror, the Destruction, the... the Evil I witnessed! How else could I explain them? Were these the Demons from my dream, or were they born... within the Wanderer?"
- "Why did I follow him...? I don't know. Why do things happen as they do in dreams? All I know is that, when he beckoned... I had to follow him. From that moment, we traveled together, East. Always... into the East."
- "We traveled East, over the mountains and into the vast deserts of broken lands. As the days passed, my companion told me of himself, that he had once been a great Warrior... and that a dark and secret burden now weighed heavily upon him. We traveled for an eternity across that arid wasteland. How long...? I couldn't say. And always, a dark cloud seemed to follow us just over the horizon. Finally, the journey ended. We climbed the last bridge. There, below us, lay our destination. The shining jewel, Lut Gholein, with the Great Sea beyond."
- "We made camp there last night. Perhaps it was the warm desert wind or the sound of the ocean, but for the first time in many weeks... I slept. However, the dreams returned, but these were clearly not my own. I beheld a vision of a great man - the Mage, Tal Rasha. You were there, too, Tyrael. I remember seeing you in my dream. His brethren had caught a Great Demon - Baal, Lord of Destruction, who had been set loose upon the world. They attempted to imprison the Demon within a sacred stone. Yet, when their attempts failed, Tal Rasha selflessly volunteered to contain the Demon within himself, completing the prison. He instructed his brethren to bind him within the Tomb, buried under the sand, there, to wrestle with the Demon for all eternity."
- "The next morning,we walked over the hill toward Lut Gholein. I had no idea then... of the horrors that were in store for me there."
- "My companion drew in the dank, cold air of the Tomb. It seemed to... strengthen him. I stood in the doorway between Light and Dark. What was left of my sanity implored me not to enter. But that voice... was just a whisper now. As we worked our way down, deeper and deeper into the Crypt, I began to see a change in my companion. He seemed to be gaining strength. I could hardly see in the gloom, but my companion seemed to know the way. We came at last to a great hall..."
- "It was then I realized my companion hadn't been gaining strength. He had been losing what was left of his humanity."
- "He moved with Demonic speed, and then... then you appeared."
- "What choice did I have? I ran."
- "I did as you told me, Tyrael. I found the Temple of the Zakarum. In the deepest recesses of the Temple, I found a dark gathering. My companion, the Wanderer... Tal Rasha... and a great Evil who could only be the Lord of Hatred himself - Mephisto. I heard a voice then... like a thousand needles in my heart..."
- "What I saw then was not meant for mortal eyes..."
- "I heard later that he was defeated, and that the Soulstones were destroyed at the Hellforge. All except one..."
- "(sobs) Oh... Oh, I failed, Tyrael... I couldn't do as you asked... I couldn't enter that gate... Forgive me, Tyrael, forgive me..."
- "Take it! Take it, take it! I'm glad this is finally over, Tyrael! Look what the stone has done to me!"
- "Baal? No... no, no, no, no, no, no, no..."
- "Oh, what've I done to deserve this..."