This mod adds a few features being (spoilers show all functions of items/entities/biomes):

Abyss

An ocean biome that contains Deepgrove Trees, Deep Moss, Abyssal Behemoths, Glow Fish, Glow Shrimps, and Trapjaw Worms. When entered water fog becomes black. Gives you an advancement if entered.

Deepgrove Trees

Generates in the abyss and comes with a whole dark-coloured wood set. Grows like Mangroves.

Deep Moss

Generates in clumps all over and even spreading into other ocean biomes around the abyss. Three types; deep moss block, deep moss slab (called deep moss carpet), and deep moss stem.

Glow Shrimp

Small Entity with 1 health that drifts around the abyss. Gives glow goo on death.

Glow Goo

Used to craft glow ink sacs and glow soup

Glow Soup

Can be eaten at the cost of getting nausea for about 30 seconds and glowing for about 1 minute

Glow Fish

Swims around.

Trapjaw Worms

Poking out of the ground they wait for prey that swim by.

Abyssal Behemoth

Giant 200 hp Entities, that can kill you in one shot if you don't have armour. Drops raw abyssal behemoth, abyssal teeth, and abyssal intestines. Gives you an advancement if killed.

Raw Abyssal Behemoth

Meat that can be cooked. Cooked meat gives you 10 hunger points.

Abyssal Tooth

Occasionally dropped from the abyssal behemoth or super rarely abyssal intestines. Can be used to craft the abyssal dagger that has 8 attack damage and 4 attack speed (swords have 1.6 and axes have 1). Gives you an addvancement if obtained.

Abyssal Intestines

When right clicked can give you slime, abyssal tooth, raw abyssal behemoth, rotten flesh and glow goo.

Mcreator was used for coding, and Blockbench for models. If you have any ideas or have found bugs feel free to say.

Known bugs (if you know how to fix any of these bugs please tell me and remember that I used MCreator):