The Holy Grail
Late in the 5th century the Knights of the Round Table discovered the secret of the Holy Grail - and then vowed to guard the precious artifact with their lives. When the Knights became old, they decided to hide the Grail to protect it from being misused - and their graves would became markers to the hiding location. The sorcerer Merlin placed strong confounding enchantments over their graves so they would remain hidden to all - however those who command powerful magic can reveal the location of the Knight's graves through the use of an enlightenment spell.
Sir Percival
Sir Lancelot stayed away from the Round Table so that he would avoid Guinevere. He met Percival, a peasant boy, and took him to Camelot to become a squire. Sir Gawain, under Morgana's influence, accused Guinevere of driving Lancelot away, forcing Lancelot to duel with Gawain to defend his and Guinevere's honor. The night before the duel, Lancelot was attacked by himself in a nightmare and awoke to find himself wounded by his own sword. Arthur hastily knighted Percival when Lancelot was late to the duel but Lancelot appeared just in time and defeated Gawain, but nearly died from his wounds. Merlin healed him and he rode out to the forest to rest - but Guinevere realised her feelings for Lancelot and they consummated their love in the forest.
Years later, Lancelot and his followers threw Perceval into a river, and Perceval then had a vision of the Grail, during which he realised that Arthur and the land were one. Perceval was able to answer the riddle, and he finds the Holy Grail. He took it to Arthur, who drank from it and was revitalised, as was the land, which sprang back into blossom.
Enlightenment Spell
The cache
You are looking for a 400mL Sistema container painted black. GPS is a bit sketchy here, please check the hint if having trouble.