rex quondam rexque futurus - "the once and future king"
"Here lies Arthur, the king that was and the king that shall be" (Hic iacet Arthurus, rex quondam rexque futurus
You were once King But lost your crown Drank a potion of forgetfulness Yet, one day shall regain it
The King within the Ancient Mountain Sleeps
The phrase "the once and future king" is said to have been carved on Arthur's grave and means that Arthur will return one day to rule and redress evil. The phrase first appears in Sir Thomas Malory's "Le Morte d'Arthur," where the equivalent Latin phrase is described as engraved on Arthur's tombstone. It's a reference to the prophecy that King Arthur will return. The idea is that he was once king, and will be again.
King Arthur is referred to as "Rex Quondam, Rexque Futurus" due to the Latin phrase "Hic iacet Arthurus, rex quondam rexque futurus" written on his tomb, according to Sir Thomas Malory's Le Morte d'Arthur.
The king asleep in mountain is a prominent folklore trope found in many folktales and legends. Thompson termed it as the Kyffhäuser type. Some other designations are king in the mountain, king under the mountain, sleeping hero, or Bergentrückung ("mountain rapture").
The Search for the Hidden King – The mystical return of the lost king archetype.
The Grail King
From Grail Knight to Grail King