Presence Concealment: A+
The greatest weapon of the Assassin class is Presence Concealment. When this power is activated, it is possible to completely cut off one's presence. When Hassan is thus concealed, it is impossible even for a Servant to sense his presence. One would not be able to tell if Hassan was approaching, if he had already left the area, or if he had even been there in the first place... Thus, Hassan is able to gather intelligence with impunity. Security systems of any kind are meaningless before him.
Also, Hassan-i Sabbāh is a being created from the nineteen people that inherited the name. When Assassin is summoned, one of those nineteen individuals will appear. Naturally, Assassin's abilities and Noble Phantasms change depending on which Hassan is summoned. However, the excellent Presence Concealment skill is a class ability, so it is shared by all Assassins. As described above, each Hassan has removed his or her nose and skin to create a faceless appearance, which carries the significance of being "nobody." It is hypothesized that the anonymity of being "nobody," the elimination of the self, also influences Presence Concealment's ability to "hide oneself." Incidentally, the nineteen Hassans each have unique personalities, so the reader should take care to remember that they are not emotionless fighting machines.
Each "Hassan" is not quite a Heroic Spirit, but rather merely a wraith (Heroic Spirits are heroes that have left their names in history, and assassins have no names). In comparison to the other Servants, Assassin's combat ability is lacking. However, the outlook changes if Assassin uses Presence Containment to acquire a perfect killing opportunity. Even if the first blow fails to defeat the enemy, Assassin can conceal himself again with Presence Concealment, and vigilantly await a new opportunity. As a matter of fact, even the formidable Cu Chulainn was easily defeated when he faced Hassan.
Nonetheless, there are weaknesses. In the moment that the user launches an attack, the rank of Presence Concealment dramatically decreases. A normal person would fail to notice the killing intent and be sent to the world beyond. However, Hassan's opponents are Servants. For example, with Artoria's Instinct, it is possible to sense the presence of a trap and respond to it. Strategic planning on the Master's part is essential to make the best use of this crafty Servant.