Schlumm, whose name means "Slumber," is a brother to Holler. He is a blind god who walks in the dream states, the meditative states, the altered states, and the subconscious and unconscious mind, etc. He is unable, though, to walk among the wakened, yet it is His greatest desire to do.
In Deitsch lore, Sunna and Muun are not siblings; they are star-crossed lovers. They don't start out as star-crossed lovers, though. The conflict in the story arises when the shadowy trickster figure, Schadde ("Shade") develops a toxic obsession with Sunna and wishes to assert his will upon Her to make Her his own. Instead of executing a simple plan and doing the dirty work himself, Schadde, instead wheedles Schlumm into doing some vile work unwittingly (Note: one might think that unwitting states of being would be Schlumm's bailiwick).
SCHADDE UN SCHLUMM
The Schadde, consumed by jealousy toward the courting of Muun, of the Wane, and Sunna, of the Ase, convinces the blind god, Schlumm, of the Wane, that there is a firm veil* that is preventing Him accessing the wakened world. Schadde suggests that Schlumm use his powerful darts† to tear into the fabric of the veil, thereby weakening it enough to allow Schlumm to pass through.
Schadde advises Schlumm on where to aim his darts, and Schlumm, not realizing that there is no veil, blows them per Schadde's guidance. One strikes Sunna, and another strikes Muun, thus casting a sleep spell over the two.‡ With Sunna and Muun now set to sleep, and the only witness being one who could not observe what just happened in the wakened world.
Schadde takes Muun and Sunna and places them into opposite locations in the skies of Mannheem, so that, when they wake up, they would be unable to consummate their love. When Sunna and Muun do awaken, they are unsure of what happened, and they ask Wudan for an investigation. The investigation yielded a suspicion that Schadde was the cause of the the attack on Sunna and Muun, and the deities call a Ding (Thing) to sort the matter out.
During the Ding, the deities notice that there is an improved stability in Mannheem because Sunna and Muun are now in their skies. Wudan asks of Sunna and Muun a great sacrifice: for the stability of the physical realm, would they be willing to stay where Schadde had put them. The two agreed, although they were sad and frustrated at the impact this arrangement has on their love.
In the Ding's ruling, Ziu sets forth a punishment to Schadde that he can never stand directly in Sunna's presence; something § must always block his view of Her.
Schadde's work is not perfect, though, and, over time, the force of gravity -- and the will of Sunna and Muun -- begin to draw them closer together from time to time, and, every so often, they meet in a solar or lunar eclipse. For the Deitsch, this is a time of revelry and celebration because two deities who made a major sacrifice for the stability of the Hatzholz (Heartwood; the physical universe), are able to share a moment in the embrace that they yearn for all the rest of the days.
This story provides a myth of how shadows came to exist, how the tides came into being, how eclipses came to exist, and possibly even devolution of faculties in the one particular instance of Schadde. This story bears some resemblance to the story of Loki tricking the blind god, Höðr, into shooting an arrow of mistletoe and killing Balder.
* The actual obstacle for Schlumm to access the waking world varies from version to version, but "veil" ("Schleier") was the most common.
† Although it does not explicitly say it in this story, other mentions of Schlumm refer to him having several herbs he uses in his darts; mistletoe is the primary.
‡ There are wide variations in whom the darts hit; one, the other, both, or even other deities or beings; however, the remainder of the story does not make sense if it did not hit both Sunna and Muun.
§ A handful of respondents here also used "veil" ("Schleier") in an unusual sense that carried a metaphysical connotation as well as representing any mundane physical object. It lead me to conclude that Schadde's sentence included a dimming of the mind's eye, so he was somewhat less enlightened. This is a brutal punishment that, ironically, might cast some light on other fragments that relate to Schadde.