Sithis

Sithus is neither Aedra nor Daedra. He is described as being everything terrible, but nothing at all; a void. A strange being, he is appeased through death, though not a god of death (see Malacath) and other acts of suffering that appease other Daedric Lords. According to The Dark Brotherhood, he is married to The Night Mother, and The Dark Brotherhood are their metaphorical children.

Groups who worship him
The Dark Brotherhood

Deepscorn Hollow inhabitants

(Un)holy Relics
The Statue of Sithis in Deepscorn Hollow