fairy stockfish

what is fairy stockfish and how do I use it?

Fairy stockfish is a engine that supports chess and chess variants, you can test it here: https://fairyground.vercel.app

How do i make a custom position to test it?

yes

Well, you have to make a variant, theres how you make it: https://github.com/fairy-stockfish/Fairy-Stockfish/blob/master/src/variants.ini