Made from the bones of a sea monster, the Gae Bolg was given to Cu Chulainn by his martial master Scathach, and tradition holds that it can only be wielded by the foot in a body of water, whereupon it explodes in a fury of blades when it pierces its victim. It must be cut out of the victim's guts to be retrieved. In some interpretations the Gae Bolg is merely a type of martial art and not a literal spear.
In video games, the Gae Bolg is often seen wielded by Cu Chulainn himself, or is an equippable item for spear-using RPG characters. Its game appearances also usually ignore it being wielded on the foot in bodies of water, probably out of convenience.