At last, I believe I have found the entrance to Naril Nagaia. It has taken some time, but I have begun to translate several inscriptions I found in the ruins.
The Ayleid script is highly advanced. The forms are morphologically mature compared to earlier inscriptions I'd found, suggesting that this place is one of the more recent ruins, from close to the height of the Ayleid power.
After deciphering several of the inscriptions, I've begun to realize that the anomalies in the writing are not, as I'd first suspected, newer, but older. The explanation for my confusion is simple: this place was indeed constructed in a more recent period, but its inhabitants were primarily ancient: this was the site of powerful necromantic experiments—from the look of things, the subjects of these experiments were the most ancient Ayleid kings!
Having made this discovery, I thought at once to return to the surface and report my findings. Only, when I tried to return to the way I came, I found it blocked. I thought for certain I was trapped, when one of the ancient spirits of this place suddenly appeared to me. He said that he had been charged with protecting the secrets of this place. All I have to do to leave, he says, is leave my book behind, and allow the memories of what I've seen to be purged from my mind.
I am still considering his offer....