The Fission Beam is a spell in Undertale Rho typically used with the assistance of a Gaster Blaster. The Fission Beam is one of the most draining spells created by Monsterkind, and is also the most powerful. The power-drain the Fission Beam has on the caster is so great that all formless, with only a handful of exceptions, are unable to cast it.

The Fission Beam is designed to behave like Level 3 Soul Magic, in that it is incapable of harming the physical environment, though functions in a completely unique way. These functions are what makes the Fission Beam the most powerful spell ever created by Monsterkind. When the beam is fired at a SOUL, the beam latches onto the individual SOUL TRAITs of the SOUL. As it passes through the body of the target, it's SOUL TRAITs travel with the beam, effectively killing the target by fissioning the SOUL into it's different TRAITs, leaving the body unscathed.

The spell is so powerful that an avatar is typically in need of being created, such as the Gaster Blaster.