Illyria's sarcophagus was the sarcophagus the Old One Illyria was buried in inside the Deeper Well. It had strange writing (probably Primordial Sanskrit) and crystals on it. It was similar to all other sarcophagi inside Deeper Well.
The sarcophagus was brought into the Wolfram & Hart offices in Los Angeles in 2004. X-rays and lasers from couldn't penetrate it.
When Winifred Burkle touched the crystal on the sarcophagus Illyria's essence entered her and eventually took over her body, Illyria coming back into being. When Illyria touched the coffin she developed a clothe-like armor.