Kronos' Sarcophagus

Kronos' Sarcophagus is the golden coffin that Kronos reformed in.

The Sea of Monsters
In The Sea of Monsters, Percy sees it on the Princess Andromeda. It is described as making the room more colder and more dark.

The Titan's Curse
In The Titan's Curse, it is at Mount Othrys.

The Battle of the Labyrinth
In The Battle of the Labyrinth, Kronos reforms in Luke's body with the coffin, with a hole in his chest. His palace on Mount Othrys is brought down on him by Nico, which stops him from leading the assault on Camp Half-Blood.