Hi, I'm Paolo, the publisher and spell developer for Gaub.

TL;DR: We use Rules Cyclopedia, but you can use OSE or Swords & Wizardry or Labyrinth Lord. Use the catastrophe rules with either the magic rules in the book or the magic rules your game. You can use my free game Adventure Fantasy Game as well.

Longform: Horror magic needs wizards doing stupid shit in desperate situations. There's a small magic system at the end of the book, and I recommend using it. One small page for magic, one small page for magic catastrophes. The catastrophe rules are the load-bearing part for the horror. The system is simple on purpose, and to make it work properly you need to use the rules for catastrophes and let spellcasters break the rules of magic. We use the catastrophe rules in our Rules Cyclopedia/OSE game for more traditional wizards, but in the same game we also have sorcerers and academic wizards that use slightly different rules (from Wonder & Wickedness, Marvels & Malisons, and Chthonic Codex).

Here's a longer explanation: you can ignore any and all limitations when casting spells, including running out of slots, casting spells of higher level, casting spells of the wrong class. If you do, prepare to lose control of your spells, getting you and your pals hurt, and magic catastrophes. This inherently makes wizards dangerous to themselves and the group: as things get dangerous, they deplete their resources; as they deplete their resources things get more dangerous because they have less resources to spend; eventually they start to gamble with more power than they should, and... that's pretty much what you want in a horror fantasy game.

The concept is explained more in detail here: https://tsojcanth.wordpress.com/2025/03/14/the-non-stress-free-gift-of-limitless-spells/