It's characterised by it's Argorrible like face and power so strong it can burn those who touch it unprotected. The stone seems to draw in the interest of a species of prehistoric like birds, who protect the stone and follow it where ever it is taken. It resides in the Nightmare Planet (circumstances always leading it to return to it's original abode).
Casted into space a thousand years ago to be lost forever, it wasn't until Zordrak, after being blasted to the Nightmare Planet by the Planet Dreamstone in "Megattack", found it in the company of it's Guardians of the Stone, took it back to Viltheed and use it as a new alternative from getting The Dreamstone. This also led him to kick Urpgor out of Viltheed, no longer in need for his inventions now he could send nightmares unaided. The furious scientist plotted revenge by taking the stone and throwing it in one of the pits in the dungeons. The plan was botched however, losing the stone that was found by Rufus and Amberley, which when given to The Dream Maker, drained all it's powers before throwing it back into space from where it came, followed by the Guardians. This was a perfect alternative for Urpgor however, with Zordrak requiring his services again now the stone was lost.
Shortly afterwards, the Nightmare Planet was found orbiting close to The Sleeping World, and on it The Nightmare Stone, now restored to it's full power. Zordrak instructs Urpgor of getting the stone, with promises of a reward if he succeeds. Urpgor sent Blob and his men to the asteroid, disguised as the Guardians, in order to retrieve the stone. Whilst in the Land Of Dreams, The Dreammaker instructs Rufus and Amberley of getting the stone, and bring it back so that The Dreammaker can destroy it. Unfortunately, they were antagonized by the Guardian birds upon entering, while the Urpneys escaped with the stone, and now had to get it from Viltheed. When the stone was given to Zordrak, Urpgor was rendered redundant and thrown out again, his "reward" being Zordrak would hold himself from turning him into stone. Blob, Frizz, Nug and the Guardians then took the stone to the tower, from which a vengeful Urpgor plans of shooting it up into space forever, but only fired up the three Urpneys and the Guardians, when Rufus and Amberley escaped with the stone by a hair, only for them to be captured and taken to the dungeons. Urpgor meanwhile was found out by Zordrak and taken to the same cell for treason, and it is from there that Urpgor instructs Rufus to drop the stone into the Pit of Storms. However by doing so had caused a whirlwind to obliterate all of the Mountain of Viltheed, and catapulted all, and collecting the orbiting tower into deepest space. But Pildit and Wildit managed to rescue to two Noops.
Meanwhile, the whirlwind crashes into the Nightmare Planet. Urpgor coyly avoids suspision from Zordrak who demands he devises a way to get him and the Urpneys back to Viltheed. In his efforts, Urpgor came across the weakened Nightmare Stone, in which he decided to keep it as his own "little secret", and with Zordrak not knowing, found a hiding place in a cave, in which he placed on an altar like Stalagmite before closing up the entrance, and from within the darkness, the stone begins to recharge. Urpgor then drew a map, so that he would remember of where it is should he come back for it in the future. However when back in Viltheed, Zordrak spots the map and thought that they were plans of getting The Dreamstone, and by reading it, and thinking that it's meaningless rubbish, scrunched it up and threw it into Urpgor's mouth, in which he accidently swallowed, destroying his remaining chance of retrieving the stone. The whereabouts of the Guardians in the episode are currently unknown.