Idea for a bit of mythology.
Each time a specific spell is cast it gains in power and complexity. Especially as the caster adapts the spell to more purposes.
Normally spell casters create a formula and only use it themselves to limit its growth.
Eventually a spell evolves to being an elemental. Instead of manifesting as a burst of energy or other more or less discrete phenomenon, it manifests as an object or creature. Usually based on the subconscious of the caster or casters. They have to be commanded to do their task or wielded but have some instincts/programming.
Eventually elementals evolve to spirits which often appear human (as their increasing complexity causes them to be viewed more anthropomorphically) like but remain soulless tools connected to the caster.
Spells, elementals and spirits can be dreadfully powerful. Especially if summoned by large groups of people. Or powered by a living sacrifice.
Spirits eventually evolve into beings at which point they gain a soul, emotions and so on. They usually have childlike emotions combined with great power and intelligence.
Patient zero supernaturals are usually former spells. For example the first vampire might have been a curse of illness. Now it reproduces by sacrificing a living person to create a new vampire (which might progress straight to spirit but take a while to become a free willed being). Yuki-Onna was a siege defense enchantment that was left summoned for a long time until she became a being. Etc.
Possible glitch for use magic includes your spell evolving. A monstrous could be a spell once overcast by the expert or spell-slinger.
A spell formula might collapse (it dies) so next time you have that effect you might need to take more time to create a new formula.
Etc.