Kronos' Sarcophagus

Kronos' Coffin is the gold coffin Kronos reformed in.

The Sea of Monsters
In The Sea of Monsters, Percy sees it on the Princess Andromeda.

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.