ishkander

The Assassins meet their end when they decline Iskander’s wine offer. He complies by using his most powerful Noble Phantasm, "Ionioi Hetairoi: Army of the King”, to massacre each and every one of the Assassins.

Lesson: Never decline Iskander’s wine offer. (but seriously, why would you want to?)

Source: http://typemoon.wikia.com/wiki/Assassin_(Fate/Zero)#Fate.2FZero