Kelp Man

The Kelp Man is the creator of the Nether, the personification of misfortune & strife, and the adversary of the Menstruation Cat. It's believed that they were once a loyal servant of the Menstruation Cat, instructed to create the Nether, a world where the Cat's mistakes would be thrown away & where sin would be kept. However the Kelp Man became infatuated with its creation & attempted to fuse the two worlds together, accidentally creating the End.

As punishment the Cat stripped the Kelp Man of their blessing & flung the Kelp Man into the ocean, where they would be entangled & emprisoned in the ocean's depths for the end of time. Afterwards two beings would be sent forth to prevent the fusing of the worlds, Pog Frog & the Ender Dragon. Pog Frog is said to watch over the Kelp Man for any attempts to excape & the Ender Dragon is to stop the Kelp Man if he ever does.

It is unsure whether the Sea Man is a follower, a version, or even has a connection to the Kelp Man as their only link to them is "The Final Step" which its main subject being about kelp, however these theories are weak and don't have much backing it.

The Festival of the Kelp Man
The Festival of the Kelp Man is meant as a celebration of the banishment of the Kelp Man, and to burn away past misfortunes & regrets alongside an effigy of the Kelp Man.