This is the writeup I turned in to Dumbledore at the end of the year to show how our spell research was going.

There are four known dark magic spells that are based upon the principles of hellfire. Three are known as the Unforgivables, and carry a guaranteed prison sentence if proven to be used. The last, fiendfyre, would probably be in the same group, but few can even cast it, and even those dark wizards that can do so fear to use it due to how hard it is to control. That seems to be why it was never similarly made instantly illegal: choosing to cast it is usually its own death sentence. All four spells resonate on an unusual frequency that makes most magical defenses useless. The only safe response is getting out of the way or allowing it to expend its energy on another target.

The fiendfyre curse is a corruption of the fire-making charm, and requires fear to catalyze. It essentially summons up an imago of pure hellfire: a terrifying animal mascot of insatiable, impossibly-hot flames. It will generally burn until it has consumed all possible fuel in reach, especially if the caster manages to survive unleashing the curse to provide continuing magic to power the imago. It's said to be hotter than dragon's fire, and can destroy virtually anything, unweaving immense protective magics as easily as it cuts through dense timber. It takes a strong caster to guide it, and an even stronger one to put it out early. Unlike the "true" Unforgivables, simply casting the spell doesn't seem to damage the caster's soul, though deaths as a result will (and it's very easy to kill many people if it gets out of control). This likely has something to do with it being the simplest expression of hellfire of any of the four curses.

The dominating curse (the imperius) requires the caster to feel spite toward the target. It essentially copies the mythological powers of the succubi, wrapping the soul of the target in the most innocent-seeming of hellish flames. It's often like being in a refreshing sauna, surrounded by embracing heat and light. The emotional decision-making parts of your mind can't come up with cogent arguments for not following the directions of the caster. Sadly, the more mechanical parts of the brain aren't incapacitated, so victims can pursue their orders with their full skill and intelligence. For those that are strong-willed or simply disagreeable, it can be broken, with less effort the weaker the caster. The biddable, however, can be subsumed for extended periods without it even being obvious to onlookers. Fortunately, only one victim at a time can be maintained by any caster; not even Voldemort can manage an army of mind control victims without an equal number of cooperators to cast the curse. Imperius "chains" are possible, however, where one victim casts the curse upon another. But it is difficult to find someone weak-willed enough to succumb but also a strong enough caster to control another target. Regardless, the strength reduces with each step in the chain until it is easy to break.

The torture curse (the cruciatus) requires an extremely potent anger at the target, even true rage. It essentially bathes the target's very soul in hellish flames, interpreted by the mind as impossible pain throughout the body that never abates. Prolonged exposure can damage the interface between the target's mind and soul, usually expressed as symptoms that seem a lot like having a stroke, even though no damage is evident in the brain itself. The fortunate may just lose some level of motor control, temporarily or permanently. The unfortunate might be driven into catatonia.

The killing curse requires actual hatred to cast, ideally for the target but research suggests that self-hatred might allow one to cast the spell (or perhaps that just slips effortlessly into misanthropy). Where the torture curse burns the soul slowly and painfully, the killing curse is like a knife of hellish plasma, instantly cutting away all ties between the soul and the body. It is in some ways an instant and peaceful way to go, because of how fast and cleanly it strikes. No one has ever provably survived being struck by it, though there are apocryphal tales of those who might be described as having robust souls not quite dying instantly.

The little-understood energy soulfire is a celestial counterpart to hellfire. While it presents, conceptually, as silver flames, they are more like a protective, empowering corona than a damaging force. Legends explain that spells directly empowered by the energy are more real than their counterparts, lasting longer and having greater strength and durability. Spells that utilize it seem to be as indifferent to magical protections as the Unforgivables, passing right through magical shields. Currently there are only two known spells that utilize this energy, though two more are in development.

The patronus charm is the counterpart to fiendfyre, and shares many similarities to light-making spells. It requires a feeling of safe happiness to catalyze, in its fullest expression generating a protector imago in the form of an animal. The summoned beast (or mist/shield for lesser castings) radiates a mood-enhancing aura that naturally drives off creatures of darkness. Fully cast ("corporeal") patronus imagos can sometimes seem to have an almost physical effect on dark magic creatures.

The empowering charm (the libertas) requires a sense of pride in the target, forging a conduit for the caster to augment the recipient's willpower. This can dispel its counterpart, the imperius. It is possible, however, that a weak libertas caster, a weak-willed target, and a strong dark wizard may prevent the curse from breaking (research pending). It also seems to strengthen the recipient's mind enough to throw off other forms of mental manipulation, like the confundus. It does not, sadly, seem to remove memory charms in most cases.

The exorcism charm (incantation-related title pending arithmancy work) should work similarly to the torture curse, but using soulfire instead of hellfire. This should have only a positive effect on those with undamaged souls, but slip into the cracks of souls damaged by dark magic and cause them psychic pain, likely through a barrage of forgotten emotions like guilt and shame. The expectation is that it should work to separate dark spirits that have attached themselves to objects or innocent beings, ending wraithly possession and similar effects where one soul or soul-like entity is latched onto another. It is currently under active development, and its catalyst emotion and incantation are not yet known.

The soulfire charm should be a counterpart of the killing curse, directing a focused blast of soulfire at the soul of the target. The primary hope is that it should serve as a way of dispatching dark entities that are immune to all mortal means of harm, such as dementors and wraiths. Any other effects are unknown at this time, as is the incantation and emotion, since all information is purely theoretical. Development is planned after the completion of the exorcism charm.