DAERN'THAL, BELIEVED TO BRING NIGHTMARES

Daern'thal is believed to bring nightmares to the dreamers, because how else do you explain nightmares? Whether that is true or not, is a moot point, as it is likely that Daern'thal controlled fire (we know this from the fact one of his priests uses a specialized fire-affinity staff) in ways that people would consider nightmareish. Furthermore, when one controls a light source, such as fire, one can also control the shadows. It would not be beyond reason to assume that Daern'thal controlled shadows, too.

He may have used these abilities to try to undermine the Evanuris, gifting them nightmares in an attempt to fuel their fall. It could be theorized that he gifted them the first seed of doubt against Mythal—and inadvertantly caused her death, which did lead to the Evanuris' downfall. But he may not have seen all of the possible outcomes.