Perron du Cerf
A block of marble in the Plessis Wood.
An inscription on the Stone of the Stag stated that marvels of the Holy Grail could be seen on the site, but that any knight – save Galahad – who stayed to see them would regret it. Yvain (Ywaine) decided to brave the adventure anyway, and awoke the next morning to find himself wounded and his two companions slain.
Source
Post-Vulgate Suite du Merlin | 1230-1240