The Glashan were formerly angels, which were made by Riathamus, aka God, and lived in Heaven with Him for a long time. Unfortunately, one of them, a Seraphim named Pyratheon, became envious of Riathamus and wanted His power for himself. Pyratheon started a war with Riathamus in Heaven and sought His power. Pyratheon corrupted many Seraphim to his side and became the most evil angel in Heaven. All the corrupted Seraphim fought against Riathamus, who was guarded by the faithful angels, until finally Riathamus caught Pyratheon in the height of the war and bound him into Hell along with all the other evil angels serving him.
Pyratheon was trapped in Hell and sought revenge on Riathamus. The angels managed to break free of Hell and go onto Earth to cause trouble, but Pyratheon couldn't leave because his chains were too tight and he was in the deepest part of Hell. The fallen angels were named the Glashan in human mythology and became the source of the world's evils.
When appearing on Earth, the Glashan use the form of strikingly beautiful middle aged men in black leather, with long flowing blond hair, and small beards. The Glashan have the power of voice mimicry and illusion casting, and deception and treachery. The Glashan know how human minds function and will play to their weaknesses of greed and anxiety.
The Glashan were released en masse from Hell when Demurral, the treacherous Vicar of Baytown, performed a dark magic ritual to summon the Keruvim, an angelic statue, to him, and this cracked open the worlds between Hell and Earth and the demons spilled out through a dimensional portal in the sky which was visible to everybody and glowed like the sunrise. The Glashan were now free on Earth to cause havoc. They set about finding officials to corrupt to Pyratheon's cause, and prepared to welcome their Lord. The Glashan also tried to kill Raphah many times, but never succeeded.
One Glashan made his home in a raven statue in Demurral's house, it had an eerie tendency to move and flick its eyes when no one noticed. The Glashan tried to kill Thomas and Kate and Raphah but when it pinned Thomas down over his heart, Thomas thought he would be killed and he entered Heaven to see Riathamus and His angels, but told Riathamus he had to save everyone. Riathamus told Thomas to strike at the Glashan in the name of the Lord, and Thomas did so, killing the demon.
However, being a demon, it was not dead for long. Demurral dragged the Glashan to the cellar and left his servant Beadle there with it.
Meanwhile, another Glashan had sought out Captain Farrel of the military who was an enemy of Jacob Crane, and the Glashan mimicked Farrel's wife's voice. When Farrel demanded it take off his blind, it did so, revealing himself, and Farrel screamed in horror at the creature before him. The Glashan ordered the Dunamez beside it to step into Farrel. The Dunamez did so and possessed him.
Back at the Vicarage, the Dunamez paid Demurral a visit in Farrel's body, and he celebrated Demurral's sorcery and how the Devil was now free. The Glashan told Demurral to bring the Keruvim to him. Then the Glashan said that he could hear Beadle in the cellar screaming for help. Demurral went down and found another Glashan, the same one thought dead, in the cellar, and it beat Beadle almost to death and ran out of the house.
The Glashan later reappeared in a frightening showdown at the church where the Glashan had taken on the illusion of an entire ward of dying people, including Thomas' own mother, in a cruel twist. The Glashan put the illusion out for as long as was necessary that the ward was normal, but suddenly the dying people became lively, turned into Glashan, and ran at Raphael and Raphah. The whole ward became full of demons which all attacked them. Raphael shouted at Raphah, Kate and Thomas to run when he held them off and to get to the church. Raphael was almost outnumbered against thousands of demons, but he held them off and finally reunited with his friends. It was never said if he killed all the Glashan or if any escaped.