Soul-Stuffing

Soul-stuffing is the layman's term for the spell, the proper name of which is long, complex and hard to remember even for the most experienced magical practitioner. It can be referenced by other names, which are easier to understand the meaning of than "soul-stuffing", such as: indestructability spell; power spell; immortality spell; power-draining spell; life-draining spell; and so on. However none of them actually convey the full meaning on their own.
The spell is forbidden by the United Nations for two reasons: one, the ritual involves human sacrifices; two, any magic that affects the soul is incredibly, beyond mortally, dangerous, not just for the person perfoming the spell but much more widely, and any spells that fall into that category are forbidden on that reason alone.
Third, any means of attempting to increase one's magical power output (as opposed to practising a skill to their natural potential) is not looked on kindly. (Generally magic is officially considered something to explore and understand but not to push the boundaries of in certain directions. This position comes out of a history of mankind weaponising everything it could get its hands on and the UN's reluctance to have to deal with the unpredictable and potentially world-shattering effects of widespread magical abuse.)
The soul-stuffing spell being cast is almost unheard of, and even rarer done successfully. For those that are successful, the consequences are enormous. Difficult to define, the best way to describe them is probably to say that the spellcaster in question can no longer be categorically described as human. They will continue to look human, and could pass unnoticed among humans, but the presence of certain magic reveals the twisted nature of the soul within.
In the presence of some magic, in fact, it is possible to see and even hear the grisly impression of the souls which the caster of the soul-stuffing spell has consumed to achieve their goal. Those victims cannot be released by mortal means. Given the immense difficulty in destroying someone who has successfully cast the soul-stuffing spell, the best solution that can usually be hoped for is containment. By definition, someone who would go this far for power, is someone who must be stopped at all costs. The most recent caster of this spell, codenamed Inferno, is currently a prisoner in the Box, the most secure magical fortress in the world.