Creatures that lived in the furthest depths of the Fundament.
On the Fundament, Savathun, one of the three exiled daughters of the Osmium King, heard instructions from a dead worm.1 Following those instructions the sisters travelled into the depths of their world where they met the Leviathan who warned them against traveling deeper.2
They ignored his warnings to meet five creatures that called themselves the Worm Gods. Yul, Eir, Xol, Ur and Akka.3
Yul described himself as being dragon-like, saying:
I am Yul, the Honest Worm.
Behold my passage. Behold my vast displacement, my ponderous strength, my great and coiling length, my folded jaws and curled wings. Behold the hiving cities symbiotic with my flesh. I am fecund, Aurash. I am at the beginning and end of lives.
Behold Eir, and Xol, and Ur, and Akka. The Virtuous Worms. Look upon us, and know that We are go[o]d.
The five creatures said that they had been trapped in the depths of the planet for millions of years, and that in that time they had summoned many species to the Fundament.3
The Worm Gods were aligned with a force that they called the Deep and opposed to the nature of the Sky, including the Leviathan and the Traveler (who they said were agents of the Sky).
According to the Leviathan, the Deep Claim was that "Existence is the struggle to exist".2 It seems that while the forces of the Sky promoted co-operation, the forces of the Deep promoted competition.
Yul proposed a bargain to the three sisters: if the sisters would take the Worm God's larvae into their bodies then the sisters would gain incredible power, enough to retake their kingdom. In return the sisters would have to promise never to abandon satisfying their curiosity, testing their strength or using their cunning.3
The sisters accepted the bargain, leading to the birth of Auryx, Savathûn and Xivu Arath.
Led by Auryx the sisters spread larvae to the rest of their species, creating the Hive. Races aligned with the Sky tried to prevent the Hive from leaving the Fundament but they were annihilated.
When the Hive met resistance against the united forces of the Ecumene, Auryx realized that way of the Deep was to take what was needed, but the Worm Gods had given their larvae freely. In order to gain mastery over the Deep, Auryx would need to take that power for himself.4
Auryx killed his sisters to gain strength (although they were not killed permanently), and found Akka the "Worm of Secrets". Auryx slew Akka and in the process learned how to take life and bend it to his will. He renamed himself Oryx, The Taken King.4
The other Worm Gods were pleased with Oryx's actions.
Prior to the events of the first Crota fireteam, Toland the Shattered said that he had heard four words repeatedly screamed across the soundless plains: Eir, Ur, Xol and Yul.5
Toland believed that these sounds, plus a fifth sound that he could not discern (probably "Akka"), held an important secret related to the Hive, and that they might refer to "Beings that once lived, or still live somewhere buried amongst us. Beings the Hive perhaps owe their very existence to."5