It's vaguely suggested that these worms/entities were locked down there by something. Then said something tasks Leviathan to guard them. And that the needle ship got them down there.

I don't understand 90% of wut u said

Okay so there's a gas giant called fundament with a bunch of moons(one of which is our traveler). Lots of races live on the moons and lots live within the gas giant on "continents" like floating chunks of land. At some point in history, the worms(hive gods, possible ahamkara) are brought to fundament and trapped in its depths. There's a very large ship in the fundament that oryx finds, referred to as the needle ship, and that may have been what brought them there. Basically oryx releases these worms and takes the ship and they teach him sword logic and killing logic. Then he proceeds to massacre everyone in the fundament and then most of the galaxy with his evil sisters. That's as abridged as I can put it, there are some awesome lore guides on the forum and on the subreddit, but nothing will teach you more than reading the actual grimoire. I hope this helped though.