A realm that shows a resemblance similar to the nether and end worlds; however this realm looks like a dark and ill realm, full of undead and has a creature imprisoned and if freed from its prison, can cause havoc on the real world as long as its alive. This thing will release tougher undead, vampires, werewolves, undead birds to spawn in the real world. To find this creature, you must begin by finding its underground temple in the real world and a artifact to open the portal to enter its dimension. By closing this portal though stops the risk of its terror at any time. The only issue is its always guarded by a creature (that always comes back every once in awhile or re spawns over a spawn block or something) to stop anyone from attempting to open/enter the portal.
Similar idea to "idea #1", but instead there is no portal, but a difficult artifact to craft that once crafted and in your hand similar to the totem of death, you die and re spawn in the undead realm with all of your belongings. In some way figure a way of getting back into your normal realm. Basically its like a second chance of being alive.
Conclusion: Basically would like to see a new realm that adds more well-known undead creatures. A boss that could be challenging to fight in a new sort of way. A new addition of enchantments and ingredients that may easy to make with this realm (SKELETAL potion = Transforms you to look undead)
ADDED THOUGHTS: Somehow make the temple almost impossible to enter until certain amount of time, skill, spawned once bought by a cartographer, certain item to open the entrance to the temple etc.
ADDED THOUGHTS: Basically would like to see a realm that is looks similar to the real world, but is all undead looking.
ADDED THOUGHTS: Would like to see werewolves and vampires and other well-known undead creatures added to the game.