Tanis is a vampire of considerable age and is the official Coven Historian. However, 300 years prior to the events that occurred within the first Underworld movie, he had been accused of recording "malicious lies" about Viktor, one of the Coven Elders. For his trouble, he was exiled by Selene to an abandoned monastery.
Marcus Corvinus stated that he often enjoyed the company of Tanis. This is most likely because Tanis had discovered the truth and thus knew who the original Vampire was all along: Marcus.
During his imprisonment, Tanis established a comfortable life by trading with the Lycans (most specifically Lucian), providing the UV rounds which are fatal to Vampires. In return, he received Lycan Bodyguards, and also wine, women and other comforts — making his exile quite pleasant. Selene believes that Tanis had to kill many women to turn the two vampire women he lived with when she went to the monastery to confront him, suggesting Tanis is willing to kill to find an attractive lover.
Underworld: Evolution Edit
It is through Tanis that Selene comes to realize her great importance and why she's valuable to Marcus — after William was captured, it was Selene's father who built his prison, and following the deaths of her entire family she was the only one who knew its location. She also holds half of the key to William's cell, which had been passed from Sonja to Lucian, and which she had also seen in her childhood but forgotten. Tanis informs both Michael Corvin and Selene about Marcus' history, and that of his brother William. He also arranges for Selene and Michael to visit Alexander Corvinus in exchange for Selene's "discretion," which she agrees to.
Sadly for Tanis, he does not survive long enough to see freedom. Marcus, who was looking for both of the keys to William's prison, arrived and interrogated Tanis. Frustrated with Tanis' lack of answers, Marcus bit him, acquiring his memories through his blood, including those of his conversations with Selene and Michael. This presumably killed him, as he is seen slumped unmoving on his table after Marcus departs.