Gae Bolg

"Barbed Spear that Pierces with Death [Noble Phantasm]
Gae Bolg.
The cursed spear that always pierced the opponent's heart after being thrust.
Reverse causality --- a technique which broke the rules by launching the spear after it was already established that the heart was pierced.
Since the target was already hit by the spear, the opponent would always be hit regardless what kind of evasive maneuvers he took.
In order to evade Gae Bolg, one would require Noble Phantasms that automatically resurrected the target after death like that of Berserker's, or a high degree of Luck sufficient to negate the curse of the spear.
To put it into game terms, it would be an insane Noble Phantasm with a base damage plus an additional component that scaled with the target's HP.
For example, if Saber's HP was 100, then Gae Bolg's damage would be (Spear Damage + 100). She would absolutely be dead if it hit.
It was also an efficient Noble Phantasm with mana consumption only in the double digits.
Anti-Unit Noble Phantasms probably did not get anymore economical than this.
By the way, Piercing Death Thorn was a technique Lancer developed himself.
Originally Gae Bolg was a thrown Noble Phantasm. It was an Anti-Army Noble Phantasm rather than Anti-Unit."

Fate/side material - Encyclopedia: Barbed Spear that Pierces with Death [Noble Phantasm], p.061-062