Gae Bolg

Piercing Spear of Death Thorns.
A cursed spear that always pierces the target's heart.
It is an unfair technique that reverses cause and effect, thrusting the spear only after the reality of the heart being hit has already been created.
Because the spear has already hit, no matter what sort of evasive maneuvers the target takes it will always reach them in the end.
In order to escape from Gae Bolg, you would either need a Noble Phantasm like Berserker's capable of automatically resurrecting you after death, or enough good fortune (LCK) to throw off the spear's curse.
Putting it in gaming terms, it's an outrageous Noble Phantasm that scales its damage based on the enemy's hit points.
For instance, if we assume that Saber has 100 HP, then Gae Bolg's damage would be (Spear Damage) + (100)! As you can clearly see, being hit means certain death.
Such convenience comes at a cost of less than a hundred units of magical energy. It's hard to believe that such an efficient anti-personnel armament exists.
Also, it's a bit of a digression, but this version of Gae Bolg is something Lancer came up with himself.
The original Gae Bolg is actually a throwing weapon, and is anti-army rather than anti-personnel.

Fate/side material: Fate Encyclopedia