“The Old Man of the Mountain” – among the people who succeeded the name of Hassan-i Sabbāh, only one had the power to switch bodies.

Differing from other Hassans, he did not need to modify his body in any way. Or, it could be said that there was no purpose in doing so, though he was typical in terms of strength, his mind was able to change his body freely.

He could use impressive planning and tactics, understand languages of other countries, identify poisons or set traps. All in all, he was a master assassin – able to do everything, and switching in different abilities based on the requirements of his assignments. It was said that on occasions he is able to utilize strange strength and agility to use illusionary fighting styles that had been long forgotten.

He could disguise as man or woman, youth or elder – anything! He could stand quite naturally next to you. Sometimes, he could even change personality based on situation so no one can guess at his real identity.

Nobody knew the truth. Hassan may have had a unique body, but he had many different souls.

The knowledge at the time could not think of multiple personality disorder as an illness.

With said definition in modern medical sciences, it was a source of arcane “power” to Hassan the assassin. He could use the multiple personalities within him to use all kinds of different skills and draw upon their knowledge, confusing their opponents or weaving a web of defense, and kill their opponents with unexpected methods that no one could predict.

This is the assassin that Kirei had summoned – "The Hundred-Faced Hassan".

He is a Servant that had one physical body, but at the same time possessed a thousand different souls. Analyzing from this basis, “they” were initially different souls in the first place, and since they are now no longer limited by the physical body, “they” can now all materialize simultaneously into different shapes.

Of course, their strength was also limited to being just one person, and after their split the Assassins cannot hope to match the other Heroic Spirits. But because they possess the unique skills of the Assassin class, they were unparalleled in terms of spying and gathering recon.

- Fate/Zero: Act 8, Part 6