Illyria was one of the legendary Old Ones - an original pure demon who ruled from her citadel (Vahla ha’nesh) in modern day Los Angeles. She was murdered by rivals and her corpse was placed in stone sarcophagus, with her powers drained and placed in jewels embedded on her coffin. Her sarcophagus was placed in a mystical graveyard known as the Deeper Well along with the coffins of other Old Ones. A warrior of good and his army were assigned to guard the Deeper Well to prevent anyone from extracting one of the coffins, as the Old Ones are capable of resurrection.

Illyria’s sarcophagus was teleported to Los Angeles, but was held up by customs officials, but Gunn was convinced into releasing the sarcophagus. Masterminded by Knox, Fred’s colleague and Illyria’s self-fashioned Qwa’ha Xahn (High Priest), the sarcophagus was delivered to Fred at the Wolfram & Hart labs. Illyria’s essence was released into Fred’s body after she touched one of the coffin’s embedded jewels.

Illyria’s essence acted as an infection that liquified Fred’s organs, hardened her skin and consumed her soul. After hours of agony, Fred’s body was completely taken over by Illyria, who in the process absorbed her memories. Illyria, with the aid of Knox, attempted to bring about the destruction of humans by resurrecting her vicious ancient army, but soon discovered that it had been destroyed long ago. Lost and without purpose, Illyria agreed to learn how to adjust to the modern world with the help of Wesley, who felt drawn to her due to his love of Fred. Spike was assigned to spend time with the Old One, to discover how powerful she was. Her powers included altering the flow of time, superhuman strength, agility and reflexes, and altering her physical appereance on a basic level.

Eventually, Illyria’s power became too much for her new body to hold. Illyria was thrown out of linear progression of the time line, altering her perspective as the power sought a way to escape. Wesley used a device called a “Mutari generator” to extract the power bleeding out of the shell, ending the threat, but this robbed the Old One of her former power. She was brutally beaten and humiliated by Marcus Hamilton, which fueled the her with anger and motivated her to join the final battle against the Senior Partners.

In the series finale, when Wesley was dying, Illyria comforted him in Fred’s form. Grieving for him, she joined the final battle against the Senior Partners.