The Imperius Curse and the Cruciatus Curse were two of the three Unforgivable Curses. The remaining third curse was the Killing Curse.
The effects of these curses could be inferred from their names.
The Imperius Curse seized control of the soul. It manipulated others... or rather, the souls of other Lifeforms. To cast it well, without letting others easily spot the flaws or allowing the cursed victim to break free from the caster's control, the caster needed exceptionally strong control.
The Cruciatus Curse sounded like a curse meant to torture other living beings, and the caster needed intense hatred. But in truth, that was not this curse's original purpose.
This magic disrupted the compatibility between another Lifeform's consciousness, such as its soul, and its vessel, such as its body. As the soul and body were torn apart, it naturally inflicted bone-deep agony upon the victim.
But during clashes of spells, or battles over the control of magical items...
Stripping away the original imprint was the purpose for which this magic had first been invented.
The Killing Curse... its origins were unknown, and it was suspected to require a contract with some mysterious existence. Furthermore, this curse carried immense risks.
After striking its target, it established a connection between one's own soul and the other party's soul.
The caster sacrificed their own soul and used it to clash head-on with the target's soul.
Of course, as the caster, one's own soul was bolstered by the curse, making it essentially capable of instantly crushing a completely defenseless target soul.
But... there were still certain magics that could block this curse, forcing the caster to pay an enormous price.
For example... the most ancient Protection Charm.
It used love to maintain one's sanity, burned the soul, and unleashed magical power.
A Killing Curse