To get to the "Mystery Realm" you need to make a portal like any other but with sandstone and instead of flint and steel you have to fill it up with coal ore this realm will spawn every 4 elemental dragons except the Fire Dragon cause thats in the nether in the Realm theres four biomes The Ocean of The Beast which is where the Water Dragon spawn there will be a crater in the middle of the biome when your in 20 blocks of range the Water Dragon will fly out of the hole. The Island Of The Skies the island of the skies will be a massive whiter plains biome with a crater in the middle it has bedrock at the bottom of the island when you are 10 blocks in range of the crater the Air Dragon will spawn and it will fly up. The Cavern Of Coal the cavern of coal is a bit like a stronghold but theres one MASSIVE room underground with a crater in the middle when your 10 blocks within the crater the Earth Dragon will spawn. Finally there is the Mysterious Plains this will be quite like the regular plains in Vanilla except it will have the attributes of the Mystery Realm.
This Realm is easy to get to but hard to handle everything there is hostile zombies,skelestons and creepers don't burn and spiders are hostile in daytime even pigs sheep and cows do 4 hitopints in vanilla in this it will 20 damage everything is more purple red and white