The Crypt is the origin point of many hostile mobs that are not Enders. This includes Skeletons, Spiders, Creepers, Zombies, Withers, Relic Goliaths and more. Due to this they exist in greater numbers and therefore this is a tough dimension to conquer.
However, some mobs are unable to cross to the Overworld, Nether, Kyther or Zenith. These include Skeleton Juggernauts which are Bone Warriors made of multiple Skeletons. Another example is the Gravedigger which is a beefed-up zombie. There is also the Treeper Ent, a type of tree which grows creepers as a fruit. When mined it will uproot itself and become a miniboss in its own right.
Its current portal material is undecided, but the portal colour would be a sickly green-yellow, that basically looks like vomit. You will spawn in a surface area that includes cobweb everywhere, ‘Spider trees’, Vines, Mud which acts like soul-sand and is a liquid. Below the surface of this dimension is a system of large caverns which house many mobs at a time. On average a cavern would be 100x100 blocks and be home to 20-25 mobs minimum at any one time. Larger Mineshafts are found commonly and occasionally house Gravediggers. Skeleton Castles, a feature akin to a Stronghold occur rarely occur and must be found with an Eye of the Crypt. This is created by using an Enderpearl gathered from another dimension (as this one has no Endermobs), and combine it with a Treeper drop and a Nether Star (Rename to Wither Star?)
It is likely that a new villager type will appear, which are bestial and hunt the other mobs, but are friendly to a player (The enemy of the enemy is my friend!)
The backstory/lore needs reworking, but essentially the Enderking has an uneasy alliance with this dimension's guardian to buy time. The boss is actually an intelligent being named The Condemner. It is a horrid combination of Creeper, Zombie, Skeleton, Wither, Relic Goliath, Spider and any other horror of the Crypt. The Condemner is the only Guardian aside from the Enderking that can actively command the beings in its dimension, making it essentially a Dictator.