This can be one of the most powerful single-target spells, if used correctly. Focusing the powers of the fallen, each corpse on the field will be drained of its last essences, effectively having its body destroyed. The essences will then form a ring, circling around the caster for a few seconds. Finally the essences will converge upon one foe, blasting them will their power. Often, this attack destroys its target as well, but this does not always happen.
The strength of this spell is determined by the number of corpses on the field. Often it will be used when there are six or more, as only one corpse makes it as effective as Belation. Three grants the spell similar damage as Vitriolic Sphere's first splash (not the damage over time), and five makes it slightly more powerful than Polar Ray. Seven or more guarantees an instakill effect, unless the target is extremely powerful.
Limiting the amount of corpses on the field is a great way to prevent this spell from being effective. Use spells that destroy to do so, like Destruction, Obliterate, or the ever popular Clear Bodies.
The idea behind this attack is from the Final Fantasy series. Tonberry enemies have an attack called Karma as well, which has a similar effect.