Horrorterror

The Noble Circle of Horrorterrors lead the mysterious eldritch creatures that reside in the Furthest Ring. These mysterious creatures seem to be getting HS 4939 (massacred by an unknown source) causing them to reach out to the players. They whisper to the dreamers of Derse, including Rose and Dave, and probably Sollux, Nepeta, Equius, Eridan, Feferi, Roxy and Dirk, and serve as a Dersite version of the Skaian dream clouds. Feferi's lusus, Gl'Bgolyb, is an emissary to them. They have some sort of mysterious agenda, but their involvement in Sburb remains unknown. They seem to be opposed to the creation of new universes, but also to the complete destruction of reality.

The Horrorterrors are inspired by H. P. Lovecraft's Cthulhu mythos and are in fact referred to as Outer Gods.

Player Contact
When a player goes to sleep after their Dream Self dies they appear to come into close contact with the Circle. So far, we have seen this happen to Karkat, Jade, and Feferi. Karkat and Jade found the experience profoundly disturbing. Both were unusually agitated and moody after awakening, and insisted that they would never go to sleep again. However, the experience did not seem to cause any (additional) mental instability or lasting harm. Karkat even prohibits the red team from going to sleep. By Karkat's account, Jade does eventually start napping again (though this is apparently more for the purpose of speaking to Dream Bubble residents than visiting writhing monstrosities). Feferi, perhaps due to her positive experiences with her terrifying Lusus, didn't expect to be upset by seeing the Circle, and indeed was not at all bothered by the experience. Certain of their benevolent nature she manages to talk them into glubbing a series of Dream Bubbles that serve as a communication tool between the trolls and the kids, as well as the dead and the living.

When Jack Noir and the Peregrine Mendicant were flying through the Furthest Ring, they seemed to come into contact with the Horrorterrors, but didn't directly interact with them or seem disturbed by their presence. It is however unclear if the injured Horrorterrors seen in HS 7161 (this flash) were slashed by one of the two as they flew by or if they were hurt through the explosion caused by.

Grimdark
The Seer of Light asked her Magic Cue Ball if the Horrorterrors were evil, and the response was HS 005539 (lines of incoherent text) (meaningless, except for the phrase "Oglog M'Rubbit," a ). She somehow went "Grimdark", a process described as going "completely off the deep end in every way." This physically caused Rose to develop darkened skin, glowing eyes and a black aura (which is at times thorn or tentacle shaped) and begin speaking in an incomprehensible language.

This can be interpreted as Rose losing control over the dark magic she received from the Circle; it could also be the result of a sudden surge of power granted her by the Horrorterrors as a response to her question on the subject of their morality. After Rose died and was replaced by her dreamself, the physical changes were not taken over by her dream self, although it remained shrouded in a black aura. The aura disappeared upon her ascension to the God Tier.

The word 'grimdark' seems to have originated in the table-top game, Warhammer 40,000.