A stone jutting out from a cliff in Cornwall.
After Tristan was caught by King Mark in Isolde’s chambers, Mark decreed that Tristan be burned at the stake. To avoid this, Tristan asked to be allowed one last prayer in a cliff-side chapel. Once inside, he jumped out the window that overlooked the cliff and would have been killed in the fall had it not been broken by the well-placed stone.
Source
Tristan | Béroul, late 12th century