Caer Sidi Magic Compendium
Magic is the skeleton key of energy. Having the ability to use magic is having the ability to convert other types of energy into an arcane state and manipulate it. So long as a sorcerer has some idea of where energy comes from, whether it be the food digesting in their stomachs or the heat of sunbeams on their face, and some idea of how energy behaves in the natural world, such as vibrations of sound or the pull of gravity on tides, then they have all they need to use magic to it's fullest potential.
There are more arcane sources that do not fit this rule; the fracturing of a soul, the spilling of blood, or the love of a mother, but these are rare or costly events and are difficult to harness. Other sources that don't conform are more unattainable than tricky, powers from things like; the maw of the void, the alignment of stars, thick cords of currents beneath the earth, each could be reached, one day, but not by us, not anytime soon.
All spells have a price, measured in magical energy, and if a sorcerer is unable to feed their spell it might very well consume other types of energy available to the caster, including the energy keeping their heart beating and their brain thinking. The cost also grows exponentially when effecting the physical world; a spell to lift a person would cost only as much energy as it would take to do physically, whereas the energy required to lift a carriage would be twice as costly as if they had physically attempted to lift it - which is fine if the sorcerer has some form of energy to draw on as fuel, but the ability to lift a house would be supremely difficult to do, certainly not something the average practitioner could do. Manipulating energy on the other hand, as in converting magic into blasts of fire, energy shields, or wiping heat from the air, these are much easier.
There does remain a few things our greatest minds have found impossible to do via magic; immortality for one. There are curses and enchantments for undeath, youth, resurrection, but no one is ever truly free from the spectre of death. Another is mind control. There are mind magics yes, and ways to read minds, converse telepathically, shield thoughts, but there is no way to magically alter or slave the mind of another. Magic can still be used to reach the same effect, stealing someone's energy to make them fall asleep, show them illusions to make them think something is happening that isn't, but directly robbing free will has been found to be impossible.
Types of Magics:
There are Rituals, Rites, Spells, and Curses.
Rituals: Used by following Ancient Codices, Occult Lexicons, or Ritual Texts. If you have the correct ritual for what you're attempting to accomplish, the required sacrifices of material or... living things, you can use it during a certain weather or astrological phenomena that the ritual is tied to, and once completed, transforms the target into the result of that particular ritual. These tend to be permanent and have few drawbacks, or it tends to go... badly if disrupted or practiced improperly.
Rites: Steps one can take to become whatever one may want to become. Many rites require determination, sacrifice, and personal experience or loss to achieve, and take a period of time.
Spells: Timed transformations that require the expenditure of magical energy, mana, or sacrifice of ritual components, but tend to have either odd quirks or short to moderately long durations, from minutes to hours at most. It is rare to find a long lasting spell, despite what the old fables say.
Curses: Transformations that act like an affliction, and are received by either cursed creatures spreading said curse, such as mummy rot from mummies, lycanthropy from were-creatures, vampirism from vampires, etc. They tend to have few upsides though some may be very powerful, but almost always have one or more major downsides.
Necromancy: Can happen with a lot of energy and powerful magic. One necromancer cannot feasibly raise 100 undead minions on a whim. More reasonably a sorceror may raise a single being to just a few at a time, depending on the strength, power and knowledge of the necromancer. Remember that the energy and power it would take to raise just one being would be quite a lot, so you'd need your most powerful necromancer to raise more than one at a time. Those instances are going to be exceedingly rare. As for undead, it's dependant on the circumstances of how they became undead. You cannot have immortal undead. Immortality is basically unfeasible in the warren, even the hardest people to kill do have at least one way they can die, even if it's tough to achieve.
At it's core, magic costs. Strong magic costs quite a bit more. So much that regular individuals are not insignificantly limited in what they can do. For example, elemental bending such as raising earth, throwing fire, gusts of wind, a giant wave to crush a ship, are going to take a lot of cost. This cost can vary. You can draw from your own life force. You can study elements and become in tune, and use will power and mental energy. You can cast spells using ingredients or read a chant from a book for a cost. But the more powerful the magic, the bigger the cost. It could be effectively inherent curses on them in return for their ability to use more magic.