A Really Deep Ocean Biome (explanation and ideas below)

An ocean biome that is really really deep, reaching the bottom of a Minecraft world.

At the deepest parts it's super dark because the sunlight can't reach that far down. Because of the huge open space that this biome has, big underwater creatures will be able to spawn and move comfortably there (whales for example but with a fantasy twist on them so their appearance is unique and special (for example dionosaur like whales), and other giant fantasy mobs (maybe inspired form real life creatures) would add to the atmosphere).

The mobs in this biome should have some sort of hunter/prey relationship between them so the biome resembles a working ecosystem.

The incentive that the player has to explore this biome is the creatures that live there. With that I mean that this is the place for the player to really get to see the megafauna and the rare animals that exist in the world of Minecraft, that were only heard in legends before or only heard as ambient sounds in the ocean waters. As a bonus incentive maybe killing or taming these mobs has benefits for the player, and maybe this biome has valuable and rare resources too.

One of the mobs in this biome should be a huge underwater predator to make things challenging.

Also some smaller ambient mobs like Nautiluses and Jellyfish or Trilobites on the ocean floor and small fish would be a great addition as ambient mobs just to make the biome feel more alive.