Illyria is a fictional recurring character created by Joss Whedon for the television series Angel, portrayed by Amy Acker. She is credited as a main character in the last third of season 5.

The character is introduced in a story where the main character, Fred Burkle, dies of a mysterious infection, after which her dead body becomes the host of an ancient demon, Illyria, who once ruled the world but now finds she has outlived her kingdom. Subsequent episodes featuring Illyria document her struggle with adapting to the human condition, Fred's memories, and the continuing grief of Fred's friends and loved ones, who make up the central cast of the show.

"Illyria raises significant questions about how essence and existence are related. This is central to understanding existentialism. Whedon raises many challenges to existentialism through Illyria's journey—from the point when she pops into existence at Fred's death, to when she 'performs Fred' as Wesley dies."

Though the character only appeared in the final episodes of Angel's final season, Illyria appears heavily in various official and unofficial comic book continuations of the show, including licensed fictional crossover stories where Illyria is the only character from the Buffy the Vampire Slayer television franchise to appear. The character has also been subject to analysis from academics, several of whom frame the story of Fred's death using feminist film theory.

Illyria is one of the legendary Old Ones, original pure demons from the Primordium Age, who ruled territory that included modern-day California. She ruled from her citadel, Vahla ha'nesh, which corresponds to modern-day Los Angeles. Illyria was eventually defeated and murdered.

Illyria had high status among the Old Ones, such that she still had followers and acolytes in the modern day. When the Old Ones became extinct, Illyria's essence was placed in a stone sarcophagus, which was kept in a mystical graveyard known as The Deeper Well along with the coffins of other Old Ones. A warrior of good was assigned to guard the Deeper Well to prevent anyone from extracting the coffins, as Old Ones are capable of resurrection. In the 20th century, the Keeper of the Deeper Well was Drogyn the Battlebrand.

Before falling, Illyria planned her resurrection. She hid her temple in a different time plane, rendering it untouchable, until she could return to open the gateway. Illyria's army was placed there as well. In the intervening centuries, the army was destroyed.

As pre-ordained, Illyria's sarcophagus is teleported to the general area of Los Angeles, but due to continental shifting, it ends up outside the United States of America. Somehow, it is delivered to American soil, but is held up in Customs on its way to Wolfram & Hart. Dr. Sparrow manipulates Charles Gunn into releasing the sarcophagus. Knox, Illyria's self-fashioned high priest and the mastermind behind her resurrection, has the sarcophagus delivered to Fred Burkle at Wolfram & Hart's science lab. Curiosity draws Fred to the sarcophagus, and upon touching one of the embedded crystals, Illyria's essence is released into her body. Illyria's spirit acts as an infection, liquifying Fred's organs, hardening her skin, and allegedly consuming her soul. After hours of agony, and despite Angel and Spike's efforts, Fred's body is completely taken over by Illyria, who also absorbs her memories.