Gáe 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

The Piercing Spear Barbed With Death.
This is Lancer’s anti-unit Noble Phantasm.
Gae Bolg is a cursed crimson spear that was feared for its ability to always pierce the heart of the enemy in a single thrust. This is achieved by reversing causality. The spear is thrust at the enemy only after the result of piercing their heart has been determined.
This power is invoked by calling the spears true name, which makes the spear possess the causal result of piercing the enemy’s heart before the spear is even thrust.
The spear doesn’t pierce the enemy’s heart because it was thrust; it is thrust because it pierced the enemy’s heart. It’s an attack that alters destiny itself.
Gae Bolg can’t be dodged merely through means of high agility. In order to evade its attack the target must have extremely high luck in order to change their fate from even before Gae Bolg is activated.
While Gae Bolg is strictly anti-unit and specifically targeting the heart, it’s actually a manifestation of the super power ability to alter destiny. As a Noble Phantasm Gae Bolg is one step away from being classified as an Authority, but that is only to be expected seeing as its wielder is Cu Chulainn, the child of a god. You can’t underestimate Lancer ever even if he seems like he is a pushover.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA