that mod is for nether, that adds mobs, items and block
this mod example like a nether is spooky, this a for minecraft nether mod
Mobs:
Pure Ghast - that small ghast that not shoots fireballs
Fire Medusa - the flying creature that immune to fire
Flying Eyeball - that spooky mob that flies around to nether
Fire Spirit - that awesome creature that spawns in nether, has special abilities, ability 1: that attacks you for around and ability 2: that shoots fireballs at you
Fortress Cow - that purple cow with black eyes, they are neutral, you attack them, they attack you
Soul Cow - that soul sand cow with white eyes, they are neutral, you attack them like fortress cow, they attack you
Items:
Eyeball - this dropped by Flying Eyeball, you have a chance to kill a flying eyeball
Ball of Spirit - Uncommon Item that dropped by Fire Spirit
Block:
Mucus - that animated block look like from picture
more blocks for sure, since this is a pre nether update mod, maybe you should add an ore like netherite. (harder to find and more powerful than netherite tho)