The Nightmare Skulls or Skulls are a race of creatures that were born out of the elven king Oberon's nightmares. The inner demons that plagued his darkest nightmares became real to turn the tide of the war with the dwarves, but at horrible price.
When the 'dreaming' started, Oberon's deepest nightmares gave birth to the Skulls, manifestations of mind made flesh. They fought off the dwarven invaders, but as soon as the dwarves where driven back to their domain the Skulls turned on the elves and killed any who had survived the dwarven invasion.
Oberon's nightmares plagued Evernight and slowly started to corrupt the once-peaceful forest. With every passing day the forest became more and more corrupted, becoming the darker and twisted version of Oberon's nightmares. The corruption even infected the purest of all of Evernight Forest's creatures, the unicorns, turning them into vicious beasts with a taste for flesh and blood.
Many of those who entered the forest to find out what happened to the elves never returned. The Skulls killed them and used their skulls and bones to construct many altars, totem poles and other horrifying decorations throughout the forest, performing unholy rituals to worship their "Father", increasing the corruption of Evernight.