See also: Spell Combinations: Nightmare

Codex text

Creatures that are asleep are particularly suggestible, and thus ill-equipped to resist the spine-chilling effect of a Horror spell. The vivid nightmares that result inflict massive spirit damage, the shock killing many lesser creatures outright. Those unlucky enough to survive are certain to emerge from the ordeal in a state of fear.

Further information

The target takes heavy spirit damage, but the sleep effect is lost. Horror still takes its normal effect after damage.