Annuvin was a wasteland of dark enchantment, and its very environment was deadly for most living things, especially Fair Folk. The closest Fair Folk way post to Annuvin was that run by Gwystyl; such near proximity to the Land of Death seemed to affect his health and temperament for the worst. Doli himself found that Fair Folk who lingered inside the borders of Annuvin would weaken and die -- until the dwarf discovered that when invisible he was immune to the effect.