Deep beneath the Galarpos Mountains, far below even the deepest reaches of the Underground Pass, lay the hidden settlement of Dinas Gwaelod, the Deephold: the secret base of a band of rebel Dark Elves, a remnant of the old Iorwerth cult of the Dark Lord, who called themselves y Ceidwaid, The Keepers. It was irreversibly destroyed in Year 8 of the Sixth Age.
In Year 2 of the Sixth Age, after the death of the old Lord Iorwerth and the destruction of the Dark Lord, most of Clan Iorwerth accepted Iestin Edern as their new leader and were reconciled, more or less, with the other Elf clans in the restored city of Prifddinas. There were some among them, however, who refused to give up their old death-cult, despite the destruction of the Dark Lord: these "Dark Elves" were never stamped out entirely - they merely learned to keep their religion to themselves.
One such sect, styling themselves Ceidwaid y Tywyllwch or "the Keepers of the Dark", was formed by a veteran Iorwerth assassin named Marwnad. For the Keepers, from the start it was never enough to practise their faith in secret: their goal was to see Prifddinas returned to Dark Elf control.
The creation of the Deephold
Marwnad sought the aid of one of the most powerful and reclusive of the old Lord Iorweth's allies, an ancient magician by the name of Mathonwy, to help her build a secret base for her sect and ensure it could not be found; but Mathonwy went one better. Over three years, Mathonwy designed and oversaw the construction of an entire hidden city for them, beneath the deepest caverns of the Underground Pass, where no one would stumble across it by accident; and he concealed its entrance behind an enchanted labyrinth that could not be solved except by one who already knew the way.
During the construction, many Keepers were required to dwell within the Underground Pass, protected from its monsters and madness by Mathonwy and their best magicians. Whenever the rare traveler strayed too near to the site, the works would fall silent, the Keepers would don their old Mourner disguises, and in most cases, the traveler would never be seen again; although on one occasion, a band of Mourners who went out to waylay a pair of travelers who strayed too near were themselves killed. The sole survivor returned babbling about a giant crystal owl, but Marwnad dismissed this as some kind of hallucination, and Mathonwy said nothing.
The Deephold was finally completed in Year 5 of the Sixth Age, and the Keepers officially took up residence there.
After Marwnad kidnapped her daughter Fynnien at the end of Year 7 of the Sixth Age, hoping to convert her into a Dark Elf, Tylluan Hen, who had lain in wait in the Underground Pass where the Keepers had previously been observed, was able to follow them undetected through the Labyrinth and reveal the location of the entrance to the Elves of Prifddinas.
A force of Elves, led by Talhaearn of the Cadarn, entered the Underground Pass by stealth, and launched a surprise assault on the Deephold. The Keepers, outnumbered and cornered, were swiftly defeated. Many of them fled or surrendered, but a large band broke through the Elven line and escaped into the Pass. A few eventually made it back to Prifddinas, where they continued to practise their death-cult in secret.
In the wake of the Cadarn victory, the enchantments Mathonwy had laid on Dinas Gwaelod were painstakingly undone by the crystal-singers of Prifddinas. Finally, the cavern was totally collapsed by their mages.