Actually I'm gonna lay out the full rules of it:
Magic = reused potential. Seeds work, fertilized chicken eggs work, both are fairly common materials to have on hand.
What you can do is restricted by the size and lifespan of the thing that would have been made. With a chicken egg, you can manipulate up to a chicken's size of material, up to a chicken's lifespan of change. So you could grow a small tree 10 years' worth of time, but you couldn't make a diamond that'd take a million years.
Dead eggs/seeds/etc won't work. You boil an egg, you grind wheat into flour, you've used its potential up for something else. Don't let them go bad.
You can use these to extend your lifespan, so long as you've gotten the proper mass ratio. Like, a chicken lives for 10 years, but it's a 5lb chicken. If you're 200lbs, it takes 40 eggs to give you an extra 10 years of life. In tree terms, an acorn can give you a few centuries, provided you got the right kind of seed; they've got more potential, but they're less consistent, because while a chicken is generally uniform in size/shape, a tree is going to be different every time.
Dragon egg = practically immortal.
People can technically also be used, but that's horrible and illegal. Use of already-living things for their potential is both considered cruel and inconsistent; a pig halfway through its life only has half left to offer, but that other half is also based on its current health. So if it's a pig with a heart problem, its potential is impacted by how much that heart would take away.
Wherever anything dies that had a lot of potential, their natural magic goes stagnant and lingers. So lumberyards, butcher houses, and warzones tend to accumulate a lot of stagnant magic. Overtime, the stagnant magic gets absorbed and repurposed, but if too much death happens in a condensed area that can't handle the magic, the area gets cursed.
Cursed areas give rise to undead. Ghosts are echoes of the potential that was lost, zombies are bodies that are trying to cling to their potential, etc. They claw for natural magic and the corruption spreads. This is most common on battlefields that saw a lot of bloodshed.
Stagnant magic is what lets people use necromancy. You can control it in order to puppet the things it clings to, or you can use it to heal things, as injuries bleed magic that quickly turns stagnant.
Turning stagnant land viable again involves an arduous process of rewilding and spending. You use up the stagnant magic, then replace it with live magic. Whichever one is in excess is the one that takes over.
