The Holy Grail

Percivale has joined a monastery and tells Ambrosius, a fellow monk, of his vision of the Holy Grail.  While Arthur is away helping a maiden that appealed for his help, the knights make a vow to ride a year and a day in quest of the Holy Grail.  Upon his return, Arthur learns of the vow and is upset, and, though he tells them they must fulfill their vows, he warns that many will not return but die. Ultimately, Galahad, Percivale, and Bors see the Grail while Lancelot is only given a veiled vision, which fact Lancelot blames on sin in his life.  Gawain gives up early in the quest.  Arthur acknowledges he was right in his prediction that many would die and states that kings cannot chase visions but must rule.

When Arthur returns from defending the honor of the young maiden, he notices a storm brewing directly above Camelot:

Returning o'er the plain that then began

To darken under Camelot; whence the King

Looked up, calling aloud, "Lo, there! the roofs

Of our great hall are rolled in thunder-smoke!

Pray Heaven, they be not smitten by the bolt."

 
Arthur foresees a storm over Camelot upon his return.  He later learns of the oath the knights have taken.  Before this quest is taken, Arthur still believes that all his knight are upholding his pure ideals; after the knight that lived return from their quest, Arthur learns that Galahad is the only pure knight he has.  Purity is the only way to see the Holy Grail, and Galahad is the only knight that faces no obstacles in seeing the Holy Grail.  Sir Bors is imprisoned and Percivale must first become pure before seeing the Grail.  Lancelot faces many obstacles before only seeing a veiled Grail.  Gawain is distracted in his quest by a group of maidens and never completes the task.  Arthur becomes cognizant of the fact the knights have not emulated their king in purity.  The storm he sees shows the turmoil that is coming to his court.  It is a storm that will break up the court and result in much devastation.
 
The above painting is The Temptation of Sir Percival (1894) by Arthur Hacker.