The Gaster Blasters are weapons used by Sans, Gaster, and Papyrus. They appear to be a skull which releases a blast of energy out of their mouths.
Attacking and Blocking: The Gaster Blasters are mainly used to attack enemies by releasing a blast of energy. It can be used as a shield to block attacks, and can even block other blasts from Gaster Blasters.
Giant Gaster Blaster: There is a much bigger variant of this weapon used by Gaster when he has 50% of his power. He is seen using this in Do or Die. This Gaster Blaster is much bigger and uses a more powerful blast of energy.
Gaster Blastermination: Sans has a special attack in which he summons a big wall of Gaster Blasters towards his enemy. This attack is powerful enough to break through shields with little to no effort. This attack, however, drains much of Sans' energy.
The iris color of the Gaster Blasters may vary per user. For example, Gaster's Gaster Blasters have red and blue irises, Sans' has blue irises, while Papyrus' have red irises.
Papyrus uses his Gaster Blasters for mostly defense but will use it for an attack on something that can’t feel remorse.
Gaster Blasters are exclusive to monsters.
When Gaster is at full power, his Gaster Blaster has purple irises.
When Sans was under Bete Noire's control, his Gaster Blasters got affected as well, which can be clearly seen, due to them having pink eyes and pink colored blasts coming out of them, instead of blue.