You can read all the information about the mod in the original forum post here. In short, the mod introduces Chemical X that can be thrown at certain vanilla mobs to turn them into tough boss fights with special rewards for beating them. There's also an enhanced snow golem to construct and a spider pig to tame!
NOTE: To obtain Chemical X, either brew an End Crystal or any drop from any of the mutant creatures alongside a Thick Potion.
Since we have primarily focussed on making modpack maker's lives easier, check out the default config file here.
Q: Will this be ported or backported to x version?
A: See our stance in 'Other Versions' below.
Q: Will you add new mutants?
A: While there were talks about adding a mutant spider as a long-term goal. The primary focus for the time being are quality of life tweaks and fixes to issues.
NOTICE: There are no plans from our side to port or backport this fork as it's exclusively developed for 1.12.2, but there are other community ports you can use instead: