The original Hassan-i-Sabbah, founder of Hashashin sect called the original Old Man of the Mountain (山の翁, Yama no Okina?). He is possibly one of the the Grand Servants, as hinted by Dr. Roman before being interrupted mid-sentence.
King Hassan comes off as quite the stern and straight-forward individual. He possesess extremely low tolerate for inpoliteness, as he almost immediately annihilated Gudao's group upon their uninvited visit to his temple.
However, he is far from being heartless, simply a man with unshakable faith and strict code of conduct. He was willing to hear his visitor's plea for aid and accept its, but only on the condition that one of the Hassans be willing to die for it.
In his view, for a Hassan to plea for aid, even from their own kind is a grave dishonor to their name and faith, as they are those most rely on their own individual strengths to preserve through trials and slay their targets. Any Hassan who grows weak and/or becomes reliant on the strengths of others must be dealt with by his blade.
Unlike all of his successors, King Hassan has been describes as one who kills his targets not physically, but by severing their 'fates' through a power similar to the Mystic Eyes of Death Perception.
Interestingly, in a manner similar to Shiki, King Hassan wields a sword that may act as a medium for his abilities. In term of actual swordplay, he is more than a match for the knights of Camelot.