Thule would be the abyss between the three primary dimensions.
Features:The sun would be almost burnt out so daylight would not give much comfort in comparison to the overworld.Mobs woujd spawn and zombies would not burn. Villages would be overrun with raiders and golems will not be able to spawn, player created or not.
How to get there and how to leave: When you go through a (nether) portal the is a 2% chance it will corrupt, turn red and bring you to thule. It would then function as a normal portal.
Mobs: Phantoms would form packs and have a small chance of summoning a brood mother with would be 3 headed have 4 wings and would look similar to the ender dragon and could land. It would spit fire charges and once landed will have a chance of laying phantom eggs.These eggs would eventually spawn a phantom before breaking.The broodmother would drop these eggs when killed which could be combined with a mobs drop to make that mobs spawn egg.
The unsworn. These would be surviving villagers who would live in mine shafts desert temples and strongholds. They would use the bartering system to exchange food for tools OR tools for food.
The Forsworn would be villagers who have pledged themselves to the pillagers side. They would be a yellowish colour and would attack players. If a golden apple us dropped near them the will become unsworn and follow you until u come across other survivors that accept you follower.
All mobs with hands will pick up the best gear they can find more often in thule. Creepers form packs and wolves are always hostile.If you attempt to summon a golem it will not become animate. Vines