Manus
Magic is used by pulling ambient energy, called manus, from Hell itself. This, naturally, damages the soul. Humans have fairly weak souls compared to demons, whose souls can be built for magical use just as their body can be built for strength. Spells are how said ambient energy is applied, focusing that ambient energy through the soul and out into the world, built to do a specific thing, like throw a huge friggin fireball or healing a wound. Of course, focusing something through the souls is painful, as mentioned, so people have a tendency to say some gibberish or gesture as they cast.
To craft new spells, one must first get their thoughts out of their head. Magus's write out exactly what the spell is and what they want it to do. Each Magus has a fairly unique process for this, some choosing to display this as a mathematical formula, some as a drawing, some even as modern art pieces. While it is possible for other mages to learn these spells, it's an incredible challenge, requiring personal guidance from the original creator.
Spells are always generated to produce a specific effect, be that effect "launch a ball of flame from my hand" or "Produce a pulse of energy from my body that accelerates healing." Once that concept has been written out an memorized, one must simply focus on it and use their soul to pull ambient energy from hell and force it through the paradigm. Once done, the effect is immediately achieved. How powerful said spell is (I.E. how big the fireball is or how powerful the slowing effect) is dependent on how much energy is focused through the soul. More energy, more hurting, more powerful spell. Said pain feels like a pit in the chest, a genuinely painful guilt.
Luckily, souls regenerate over time. Botched spells also generally fizzle out and the hell energy just disperses back into the air. Doesn't mean they don't still hurt though. Soul expended on botched spells takes a toll on the soul, and even leave "soulscar," hampering the soul's regenerative process.
manus can be solidified. This allows both 'enchanted' items (Items made with manus in them, giving them special effects like a boomerang that always returns to its user) and 'charms' (items that, once broken, release stored manus energy, like a paper tag that, when torn, generates an explosion.) These are both very serious sub disciplines of magic, and huge businesses. After all, one doesn't need to harm their soul to use either of these, while still getting use of magical effects.
Manus, truly, isn't actually magical. The Soul's ability to manipulate it is. As such, Manus can't move through objects, or directly affect someone without endangering them. One could create a buff spell that increases rapidly increases muscle generation in an area, or to whoever is hit, but trying to directly affect a being's cells is impossible, instead making a horrifying mess. This is why enchanted items must be forged with Manus, and can't be enchanted afterwards.