This is not an API, but a content mod.

The following APIs are available:

• Spell Engine https://github.com/ZsoltMolnarrr/SpellEngine
  • ⚠️ Unstable API, mostly undocumented
  • Used for creating spells, spellbooks
• Spell Power Attributes https://github.com/ZsoltMolnarrr/SpellPower
  • ✅ Stable API, documented
  • Used primarily for creating armor and weapons pieces with Spell Power bonuses, so spell classes can use them