The Undergarden is a massive world hidden deep below bedrock that you can only enter via a stone brick portal. The portal is made in the same shape as a standard Nether portal only made with stone bricks instead of obsidian. You will also need to craft a Catalyst in order to activate the portal. The Catalyst is fairly inexpensive to craft and requires one diamond, four gold ingots and four iron ingots. Then once you have the portal frame built you will need to right click the opening with the catalyst like you would with flint & steel on a Nether portal.
Then simply walk through and you will be transported to the ominous Undergarden realm. This new world is indeed 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 fifteen new biomes you travel through. There are also more than ten new music tracks to help set the mood of this world without a sky.
You will 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. It is a fluid not unlike how lava flows in the Nether and it is just as dangerous but in a much more toxic way. There are vast pools of it all over as well as random patches raining down from above.
There are also vast forests full of new types of 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. There are more than ten of these new creatures lurking about and each have their own unique abilities and strengths. Some of them are friendly and even cute, while many others are significantly larger and certainly have no patience for outsiders who came to this world to visit. Definitely come prepared with weapons and armor as well as food.
But this mod does have you covered and you can easily defend yourself if you find the ores associated with new armor, weapons, and tools that can be crafted here. Each of these new ores have their own unique strengths and properties when crafted into weapons and armor and they will definitely help protect you from the unknown and mysterious adventures that lie ahead in the Undergarden. It is an incredibly detailed dimension that seems to be designed for those that want a more challenging experience. Everything here is so much different than any of the standard Minecraft worlds so it is absolutely a brand new adventure to take on. All of the unknown monsters will have different ways to defeat them if they are indeed hostile and with each new biome or cave or structure you find here will require a great deal of knowledge and strength if you want to make it back home alive and with any of the new items you happen to find.