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.
Uther Pendragon
Merlin agreed to help King Uther Pendragon seduce the King of Cornwall's wife - on the condition that he give Merlin whatever results from his lust. Nine months later, Merlin took Uther's son Arthur as his own. Uther pursued Merlin but was mortally wounded by Cornwall's knights. Before he died, Uther thrusted his sword Excalibur into a stone, crying that "Nobody shall wield Excalibur, but me!", and Merlin proclaimed that "he who draws the sword from the stone, he shall be king."
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.