Cassius is a Vampire and a Council Member of the Eastern Coven following The Purges. Not much is known about his past but he could have been one of the original Death Dealers because of his potential age and his knowledge of Vampire and Lycan history.
Cassius is the leader of the Vampire Council of the Eastern Coven and despises Selene for killing Viktor. As a result, Cassius seeks her death and is disgusted at Thomas's request that the Council gives Selene clemency in exchange for helping them to fight Marius and his Lycan army. However, reluctantly seeing the sense in Thomas' argument, Cassius agrees to the deal but leaves Semira personally responsible for Selene.
After Selene apparently murders the Death Dealer "raw recruits," Cassius is enraged, seeing it as proof that his suspicion of Selene was correct. Cassius orders Selene hunted to the ends of the Earth, but orders her captured instead of killed. He also takes Selene and David's ability to "daywalk" as proof that they are colluding with the Lycans.
Semira later approaches the Council with the news that the Nordic Coven has been destroyed and urges the Council to wage war against Marius and his Lycan army. Cassius refuses, believing the Eastern Coven to be impregnable and Semira uses the opportunity, backed by Varga and his Death Dealers, to overthrow the Council, claiming that if they won't take action, it's her duty to do so. However, at that moment, David returns and reveals to the Council that he is the son of the Vampire Elder Amelia and thus the rightful leader of the Eastern Coven. David provides the Council with vials of Amelia's blood left behind by Amelia to back up David's claim. Drinking the blood, the Council gains some of Amelia's blood memories, proving David's claim. A stunned Cassius announces that David is indeed Amelia's heir and the last of the pure-bloods and the Death Dealers turn on Semira, who is locked up on David's order. Taking command, David warns the Council that Marius and his army are coming, causing an enraged Cassius to comment that Semira will get her war after all.During the battle with the Lycans, Cassius and the other Council members fight alongside their Death Dealers but are cornered by the Lycans' superior tactics. As the battle appears lost, Selene returns with enhanced powers and the Nordic Coven for backup. As Selene battles Marius in the training room, Cassius and the other Vampires battle the now-transformed Lycans in the Eastern Coven entryway. Cassius handles himself well in the battle, at one point defeating a transformed Lycan in hand-to-hand combat before executing it with a shot to the head at point-blank range. Moments later, David and Selene return with Marius' severed head. David uses the death of the Lycan leader to order them to retreat, ending the battle.
Following the battle's end, the Vampires treat the injured with goblets of blood from three urns. As a symbol of conceding power, Cassius serves blood personally to David, Selene, and Lena, the three new Vampire Elders. While doing this, Cassius is the first to bow down to Selene as one of the Vampires' new leaders, forgiving her past transgressions and fully accepting her new role in Vampire society.
Not much is known of Cassius's relationship with Semira beyond the fact that he seemed to tolerate her as a member of the Vampire Council. However, he didn't appear to like her much, especially due to her desire for open war with the Lycans. When Semira tried to take power, Cassius was shocked by her actions and disgusted by the fact that Semira would get her war since the Lycan army was coming.
Due to Selene killing Viktor, Cassius hated and distrusted Selene. Cassius wanted Selene dead for her actions and was highly reluctant to trust her to fight against Marius despite the need for her skill and power. He only agreed reluctantly and ordered Semira to keep an eye on Selene. After Selene supposedly murdered Thomas, Cassius took it as proof that he was right about Selene all along and ordered her immediate death.
After the defeat of Marius's Army and the exposing of Semira's treachery, Cassius lost his animosity towards Selene and came to trust and respect her. As a sign of this, he personally handed her a goblet of blood as a sign of conceding power to Selene as one of the new Vampire Elders and was the very first to bow to her.
Cassius had a great respect for Viktor as seen with his reaction to Viktor's death and the fact that Selene killed him. He appeared to be unaware of the truth of why Selene killed Viktor as all he commented on was that Selene had killed someone who had cared for her so much. Viktor's death was enough for Cassius to order Selene's death with only the grave threat Marius posed causing him to even consider pardoning her for her actions.
Cassius' relationship with Marcus is completely unknown. However, he only wanted Selene dead for killing Viktor, indicating that he either didn't care for Marcus as much as Viktor or knew the truth surrounding his death and it was enough for him to forgive Selene for killing Marcus.
Like all Vampires, Cassius contained a respect and admiration for the Elder Amelia. Cassius was stunned to learn that Amelia had a son, but instantly respected her wish for David to be her rightful heir after receiving proof of it.
- Considering that he lead the Vampire Council and he's been stated to be stronger than Semira, it's very likely that Cassius is currently the oldest and, after the new Elders, the strongest Vampire in existence.
- He is one of the few Vampires who would have known the truth about the origins of the Vampire and Lycan species to have survived both the events of Underworld: Rise of the Lycans and the destruction of Ördögház.
- Given his age, Cassius may have been one of the original Death Dealers.
- "The same Selene who murdered Viktor?"
- "If she can be found... and sets foot within this precious Coven... I will hold you, Semira, personally responsible for controlling her."
- "A known traitor within our walls. This is precisely what I foresaw and what you assured me you would prevent. Thier blood is upon your hands."
- "Do not speak to me. There can be no mercy. I want her hunted to the ends of the Earth."
- "Selene must be wiped from the pages of our proud history."