O.k. We all know the King under the mountain referes to the Lonely mountain from the Hobbit. There is another possibility to the mean of it in Halo: First Strike. I came across an article on Wikipedia regarding the sleeping hero legend, namely about a king under a mountain.
"Frederick is the subject of a sleeping hero legend. He is said not to be dead, but asleep with his knights in a cave in Kyffhäuser mountain in Thuringia, Germany, and that when ravens should cease to fly around the mountain he would awake and restore Germany to its ancient greatness. According to the story his red beard has grown through the table beside which he sits. His eyes are half closed in sleep, but now and then he raises his hand and sends a boy out to see if the ravens have stopped flying."
The article can be found here:
Just my 2 cents