An alternative to Three Concealments and Three Disclosures of the Island of Britain:
First the head of Bendigeid Vran ab Llyr, which Owain the son of Maxen Wledig buried under the White Tower (White Mount) in London, and while it was so placed no invasion could be made upon this Island; the second was the bones of Gwrthevyr the Blessed (Vortimer), which were buried in the chief harbour of the Island, and while they remained there hidden all invasions were ineffectual. The third was the dragons buried by Lludd ab Beli, in the city of Pharaon, in the rocks of Snowdon. And the three closures were made under the blessing of God and his attributes, and evil befel from the time of their disclosure.
Gwrtheyrn Gwrtheneu [Vortigern], disclosed the dragons to revenge the displeasure of the Cymry against him, and he invited the Saxons in the guise of men of defence to fight against the Gwyddyl Ffychti; and after this he disclosed the bones of Gwrthevyr the Blessed, through love of Ronwen [Rowena], the daughter of the Saxon Hengist.
And Arthur disclosed the head of Bendigeid Vran ab Llyr, because he chose not to hold the Island except by his own strength. And after the three disclosures came the chief invasions upon the race of the Cymry.
See also
Three Concealments and Three Disclosures of the Island of Britain | The Legend of King Arthur
Sources
Red Book of Hergest | C. 1425
Text of the Mabinogion and Other Welsh Tales | John Rhys, 1887