No Second Strike

Mu, nida. No Second Strike.
Assassin’s anti-unit Noble Phantasm. At the very start of the battle, in the first turn, this instant death attack “brings its target to an end using the full scope of the heavens.”
…is what it’s supposed to be, but in the game due to Assassin’s artery blood flow being disrupted during the moratorium, his instant death attack is reduced to deal 90% of the target’s HP in damage.
In close combat this skill is on the same level as Lancer’s Gae Bolg, and can be said to be one of the most efficient ways of ending a battle.

Historical records say that even attacks that Li Shuwen intended as feints or diversions would easily end the life of whoever took his hard hit.
“Li Shuwen needs no second strike (God Spear No Second Strike).”
The Noble Phantasm No Second Strike is the materialization of his title.
It is technically not a Noble Phantasm, it is the quintessence of martial arts.
Li Shuwen is a master martial artist, and it goes without saying that he is extremely powerful, but the most important point for the realization of this skill is to have experience in “consuming your opponent using your Ki.”
One theory states that Li Shuwen did not just defeat his opponents using the power of his fist alone.
It goes on to say that those who were killed by Li didn’t die from having their organs destroyed, they died from what would be described in modern times as death from mental shock.

The technique of “consuming your opponent using your Ki” has no proper fixed name, and is just an idea that is slightly similar to the Ki ideologies of Sennin.
The idea is that you surround your mind and body with energy (Ki) and use it to activate your entire body through resonance and amplification.
One of the possibilities is completely filling your body with Ki, or even filling the area around your body with Ki.
Using this method Li Shuwen “consumed his opponents with Ki,” which probably means that he essentially filled the space around him with his Ki and completely made it into his own territory. The “person consumed by Ki” would have some of their senses blinded and would become nervous and stressed. It this state of mental duress, if someone where to receive a direct blow to their nervous system, the vagus nerve reflex reaction would stop their heart.
In short, death from shock.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA