It is a dimension you get to by beating the warden and going through the portal. The way to light the portal is by killing the warden, which gives us a reason to do so.
When you get to the new Dimension, it looks very mountainous and mysterious, with lots of trees and overgrowth as well. There’s a new ore that’s even stronger then Nephrite called the crystal ore, but also harder to craft as well. There are new structures that are broken down ruins of towns and villages all over the place, and all of them are covered in skulk and at least one warden at every ruins. Mobs like Enderman, Creepers, Zombies, Skeletons, and Spiders are here as well, however they are all overtaken by skulk, like an infection has come over everything. I think the ground shouldn’t be made of skulk however.
When you are walking around there’s a chance you’ll come across a new boss made completely out of skulk. The boss is as tough as the ender-dragon HP wise, and has all the same moves as the warden but quicker and a few more moves like spawning skulk Creatures (including the warden), healing if your not quick enough and sending out a shockwave on the ground that you have to jump over.
After beating him, you get two new items, one that allows you to send out sonically-charged shrieks like the warden, and a second increasing your health by double(Second one’s optional).
That’s all I’ve come up with, and I want to leave the rest to the creators to come up with more things of their own.