The legendary demonic spear of Cú Chulainn. In Fate, its the noble phantasm of Lancer, once activated, it can alter the destiny of it's target and always strike the heart.
Being the Noble Phantasm of Lancer for the fifth Holy Grail War, Gae Bolg is a cursed spear that is unique in that there are two properties of the spear that can be exploited by its wielder for close quarters and ranged combat, both activated by Lancer calling out the name of his spear.
The close-quarters attack of Gae Bolg is concentrated on a single target as it reverses causality to always have the outcome of its thrust "piercing the enemy's heart", fatally wounding the enemy by stabbing them through the chest and being unable to easily heal the wound and its effects so long as Gae Bolg remains in the world. The use of Gae Bolg in this way consumes little mana from Lancer, meaning he can perform this as many times as he needs to in order to slay his foes. Avoiding the spear's fatal attacks requires one to have a high level of luck or divine protection as such to alter one's fate against it or to quickly get out of the spear's effective range. Such luck would involve one having good instincts, an auto-resurrection ability or having a shield capable of overwhelming the spear's magical energy. Even if one is capable of avoiding the fatal attack of Gae Bolg, the spear's effects will still make one incapable of easily healing wounds sustained from its attacks for a prolonged period of time. The effect of the spear is such that it took weeks for Saber to fully recover from an attack by the spear and it took Shiro half a day to recover via use of Avalon's powers.
The ranged attack of Gae Bolg lacks the effect of its close-quarters attack variant to "always pierce the enemy's heart", yet is a more powerful attack as such that it can take out multiple enemies at once. There is a greater amount of mana put into this ranged variant as Lancer puts all his energy into the spear and releases it into a thrown attack that will detonate upon impact with its target. He normally prepares this attack from at least 100 meters away from his foe(s) and then tosses it when at least 50 meters from his target after initiating a charge towards them. Rather than focused on accuracy as its close-range variant, the ranged variant of Gae Bolg is concentrated more on attacking any enemies within a concentrated area in a carpet bombing fashion. This makes the attack very difficult to dodge or block under normal circumstances due to its greater destructive power as attempting to evade the attack will only result in its enemy targets being sent flying from the aftershock of the spear's detonation upon impact with its target.