Not even Excalibur saves a king from his destiny.
A sacred king, according to Frazer in The Golden Bough, was a king who represented a solar deity in a periodically re-enacted fertility rite. The king was annointed in the spring, reigned during the summer, and ritually died at harvest time. The King was reborn at the winter solstice to rule again.
The spirit of vegetation was therefore a “dying and reviving god” : Osiris, Adonis, Dionysus, Attis and many others. The sacred king, the human embodiment of the dying and reviving vegetation god, was originally been an individual chosen to rule for a time, but whose fate was to suffer as a sacrifice, to be offered back to the earth so that a new king could rule for a time in his stead.