The Undergarden is a massive world hidden deep below bedrock that you can only enter via a stone brick portal. You will also need to craft a Catalyst in order to activate the portal.
Once in you’ll be dropped into a strange and sometimes hostile, but overall beautiful, realm. There are particles floating around almost every step you take and they change depending on which of the 9 new biomes you travel through.
You’ll likely come across a purple liquid flowing from above or in some sort of bog. This is a poisonous fluid called Virulent Mix. Best to avoid it until you know what to do with it.
You’ll also come across vast forests of new trees and thus new materials to build with. Each new forest is vastly different in color but blends incredibly well between biomes.
And of course this mod introduces a hefty amount of new mobs both hostile and friendly, as well as small and very large.
But you can easily defend yourself if you find the ores associated with new armor, weapons, and tools that this mod adds.