Belisarius Arius is an Imperial assassin and Speaker for the Dark Brotherhood. He spends most of his time doing administrative duties within the Brotherhood. When other members of the Black Hand were systematically murdered, he and the remaining three members mutilated Lucien Lachance as they thought he was the traitor.
He was murdered by Mathieu Bellamont, the real traitor in The Night Mother's Crypt, in the year 3E 433.
After Lucien's death by the Black Hand, Arquen suggests that the Hero and the remaining members meet with the Night Mother to discuss the next steps for the Dark Brotherhood. The Hero are to speak with Arquen between 12 am and 3 am in Applewatch. After the Hero speaks with Arquen, she teleports the Speakers to the Night Mother's Crypt in Bravil. The Speakers go into the tomb and meet with the Night Mother to decide who the new Listener will be.
Just as the Night Mother is about to decide, Mathieu Bellamont grabs his dagger and kills Banus Alor and Belisarius Arius. Arquen and the Hero later kill Mathieu and the Night Mother names the Hero Listener and makes the Blade of Woe more potent than it originally was and teleports the two back to the Cheydinhal Sanctuary.