Warning! The following will contain spoilers for Angel - Season 5!
Illyria is one of the legendary Old Ones, a being with intense power who ruled over what is now present day Los Angelus until she was murdered by one of her rivals. Upon her death, her powers were drained and sealed in a jewel which was placed over her body in a stone sarcophagus. The sarcophagus was then placed in The Deeper Well, along with thousands of other Old Ones, to await a time when Illyria could rise again.
You see, before falling, Illyria planned her resurrection. She hid her temple in a different time plane, rendering it completely intangible, until she could return to open the gateway and enter it. Illyria's army was placed there as well, waiting to be raised once again by its leader. (Sometime in the passing centuries, the army was destroyed, leaving only dust in its place, but that is neither here nor there).
As pre-ordained, Illyria's sarcophagus is teleported to the general area of Los Angeles, but due to continental shifting, it ends up outside of the United States of America. When Illyria's followers attempt to bring it closer, it gets stuck briefly in customs before arriving at Wolfram and Hart, where Illyria's essence is injected into one of the employees there, and giving birth to the Old One once again.