The Underworld

Note: Current idea to reach this dimension is by drinking a certain potion.

The Underworld is a dimension where all the dead roams and lost things gather. Nearly everything you see here are ghosts, souls, memory fragments, and lost or broken things. The dimension itself is black and white, like a photo taken long time ago. It is said that the souls of the dead are sent here first to be judged if they will go to heaven or hell. Some also said that heaven and hell does not exist and this is where all the dead will go.

It is recommended that the player brings a sword with Smite Enchantment on it as it is the only thing that could damage the souls. Why come to this dimension? Not only there are Boss Mobs to fight, this dimension is also an Xp farm with the Memory Pieces dropping experience orbs and Souls giving more experience than any regular mobs.

Landscape
The terrain is similar to The Nether and The Overworld combined. The sky is covered by blocks that made the ground with the outermost layer being Bedrock (Just like the Nether). The land is just like the Overworld, flat with some highs and lows and occasionally some hills and holes.

Blocks

 * Fragmented Memory Block - A block which is colored black, white and grey. Dominate the dimension like Netherack in The Nether.
 * Broken Item - An aesthetic block which spawns a lot in The Underworld. Consists of broken Minecraft items.
 * Underworld Brick - The block which some of the structures are built out of. Looks like black, white and grey version of Stone Bricks.
 * Underworld Log - The block which some of the structures are built out of. Looks like black, white and grey version of normal Logs.
 * Underworld Plank - The block which some of the structures are built out of. Looks like black, white and grey version of normal Wooden Planks.
 * Ghostly Water - Instead of Lava, this occupies most of the lower level terrain in The Underworld like Lava in The Nether. It acts just like water, except that it looks creepy and produces creepy sound. Do not stay in it too long while wearing armor, as it will slowly corrupts your armor and turns it into a Haunted Armor. Does not affect Soul Stone Armor set.
 * Soul Fire Gem Block - Instead of Glowstone, this block will glow and spawns in The Underworld like Glowstone in The Nether. Drops Soul Fire Gem when mined.
 * Charon's Statue - A statue which spawns very rarely near the water. When interacted with, a GUI will pop up, asking the player to pay some Gold for a boat ride to the Castle of The Underworld.

Ores

 * Memory Pieces - Found in The Underworld like Nether Quartz in The Nether. Only drops exp.
 * Soul Stone - Technically this isn't an Ore but a block. Used to make the Soul Stone Armor Set and weapons

Mobs
Note: All "Soul" mobs can only be harmed by a weapon with Smite enchantment on it, magic and potions or the Soul Stone Blade. Arrows will just pass through them as they don't have a physical body
 * Corpse Retriever - Neutral like Zombie Pigmen in The Nether. Will attack any Broken Souls in order to "Repair" them
 * Lost Soul - A soul which had no memory of its living past. Passive to the player
 * Broken Soul - A broken or incomplete soul of a once living entity. Hostile towards everything
 * Entity Soul - Ghostly version of vanilla Minecraft mobs. Will attack the player if the form it takes is a hostile mob (Zombie, Creeper, etc)
 * Fiery Soul - This soul is burning bright with energy and will set any nearby flammable entities on fire. When attacked, it will explode, leaving a fiery crater at the spot
 * Haunted Armor - Spawns when a player's armor is soaked too long in Ghostly Water. The armor will break away from its owner and come to life
 * Human Soul - A soul of a dead player. One will spawn at the spot the player died while in The Underworld with his skin as texture
 * Bone Wyvern - Skeleton version of the Ender Dragon, only a lot smaller.

Note 2: All "Soul" mobs drop more exp than regular mobs with the exception of Lost Soul.

Items

 * Soul Stone Armor Set - While wearing, grants you immunity to Wither and deals damage to any Undead mobs nearby
 * Soul Stone Blade - A sword which can destroy any Undead mobs (Not including Boss Mobs) instantly but deals damage equal to a Stone Sword to any non-Undead mobs
 * Scythe - Crafted from Soul Stone and Iron Ingots. When used on a living non-Undead mob, it will pull the soul of the target out and the target will lose it's AI and interaction (Trading, etc). Does not work on Bosses, Undead mobs or any Underworld mob
 * Soul Fire Gem - Craft 4 together to make a Soul Fire Gem block. Can also be used as fuel in the furnace.
 * Soul Fire Ring - A ring that is glowing with energy. While in hotbar, it will provide a portable light source that follows the player. While in The Underworld, it reduces the detect range of Hostile Souls as they will mistake you as a Fiery Soul. Crafted from Soul Stones and a Soul Fire Gem.
 * Spirit Lantern - The lantern held by The Reaper. It absorbs Lost Souls and serves as a portable light source. When used, it will fire a soul projectile at the direction your cross-hair is facing. Must have absorbed Lost Soul in order to use this.

Structures

 * Memorial House - A house in the memory of a soul. Comes in different variation: Complete, Broken or Burnt-Down. Burnt-Down ones always contain a spawner for Broken Souls while complete ones has a small chance to contain a spawner for Broken Soul
 * Memorial Castle - A castle in the memory of a soul belonging to a royal. Has same conditions as the Memorial House
 * Reaper Fort - A fortress where the Reaper Resides
 * Soul Henge - Strange structure made of Soul Stones and Underworld Brick. This is where the Soul Magus resides
 * Castle of The Underworld - This structure only spawns in a small different dimension (Like Anu's Castle in Fossil and Archaeology Revival Mod) only accessible via Charon's Statue. This is where the final boss of The Underworld resides.
 * Boat - The only place where you can find Charon's statue. Spawns on water surfaces.

Boss Mob

 * Reaper - Grim Reaper? Yeah, only much more weaker. Could control any souls and turn them against you
 * Soul Magus - A short guy wearing robes. Don't be fooled by his tiny looks, as his power is beyond calculation. While fighting, he will absorb any nearby souls and launch ghostly projectiles at his challenger. Before dying, he will gather a lot of souls and prepare them for a final attack. Cannot be interrupted unless you destroy his arm
 * Lord of The Underworld - The final boss of The Underworld. To encounter him, you have to find a Charon's Statue first. Some says that he is Hades himself.