Underworld

The Underworld is a location which is located 1000 km under the Surface. In appearance, the Underworld appears to be a hell-ish world with red ground namely "Bloodstone" with some distant lava oceans. Due to the hot temperature (except in Warped Forests), many creatures here evolved to be immune with fires.

Pigman
Bizarre humanoid pigs, these creatures roam freely in the Underworld Wasteland and Crimson Forests, albeit the non-evolved Pigmen tend to avoid Warped Forests and Undead Valley. Most evolved pigmen live in the Pigman Kingdom (later U.H.C), led by the Pigman King, while some live independently as nomads.

Piglin
Another species of the humanoid pigs, Piglins are more advanced than the Pigmen, being able to forge their own material, Piglinium, which are even stronger than Titanium. They live in the southern side of the Underworld, separated from their other cousin by the extensive Undead Valleys.

Ocghostpus
Bizarre floating heads with several short tentacles, these creatures live in the Undead Valley, albeit some stray further from home, reaching the Wasteland or Forests. Both the Piglins and Pigmen utilize them as aerial transportation and weapon.

Wythra
Bizzare limb-less creature with three heads, these creatures roam in groups in the Undead Valley, sometimes straying further to the Wasteland. They have the ability to heal themselves as long as their middle heads are still intact and shoot skulls which can wither living beings.

Magma
Weird cube-shaped creature, the Magmas are one of the most advanced civilisations in the Underworld, alongside Piglins and Pigmen.

Hoglin
Abnormally large boar-like creatures, the Hoglins roam in groups in the Crimson Forest and tend to avoid Warped Forests and Undead Valleys. They're common in the south, but are very rare in the north.

Flaemoth
Abnormally large moth-like creatures, the Flaemoths give a charismatic environment in the forests.

Soul Skeleton
Living skeletons fueled with souls, these creatures roam freely in the Undead Valley under Wither Marrowkai's control.

Withered Skeleton
Black, tall, living skeletons that guard the Wythra God's Lair, to ensure it won't outbreak again. They'll attack any intruders that come to the lair.

Underworld Wasteland

 * Pigman Fortress
 * U.H.C. Fortress

Undead Valley

 * Wither Marrowkai's Cave
 * Wythra God's Lair

Trivia

 * It is heavily based off The Nether from Minecraft.
 * The Underworld Wasteland is based on Nether Wastes;
 * The Undead Valley is based on Soul Sand Valley;
 * Ocghostpus is based on Ghast;
 * Wythra is based on Wither;
 * Soul Skeleton is based on Skeleton;
 * Withered Skeleton is based on Wither Skeleton;
 * Magma is based on Magma Cube;
 * Wythra God is based on Wither Storm.