In The Silver Chair, Rilian, while enchanted, said:
Though under Earth and throneless now I be,
Yet, while I lived, all Earth was under me.From which it is plain that some great king of the ancient giants, who lives1 buried there, caused this boast to be cut in the stone over his sepulcher;...
-The Chronicles of Narnia: The Silver Chair, chapter 10
And before, when they are traveling to the Underworld:
"That is old Father Time, who was once a king in Overland," said the Warden. "And now he has sunk down into the Deep Realm and lies dreaming of all the things that are done in the upper world. Many sink down, and few return to the sunlit lands. They say he will wake at the end of the world."
-The Chronicles of Narnia: The Silver Chair, chapter 10
Now, Time fits the description of the scene that Rilian gave: He's a giant, he was a king, and he is located under the carving.
Was Father Time the one to have the message carved in the stone?
1 Yes, it says "lives buried" in my version.