Thergon was a coral city on the southern island of the Whisper Isles, and it was Xynoth Azkonor's before it was anyone's ruin. He was its king, and by the account that matters he was a good one, until he died. What happened to him after that death belongs to his own entry. What happened to Thergon is this: the king the city had mourned came back wrong, and made the city pay for it.
When the priesthood of Thergon refused to bless the thing that had returned wearing their king, Xynoth killed the high priest and took the city by force. He raised his first army out of Thergon's own dead, the same people who had filled its temples a season before. The living who remained served him or were added to the host. By the time word of it reached the mainland, Thergon was not a city with an undead problem. It was an undead city.
The Pity Knights came for him because he had been one of theirs, a sworn Knight who had turned the order's own lich-craft on himself. They could not kill him. A lich that has split its spirit as many times as Xynoth had is a hard thing to end, and he is hard to end still. So they did the other thing. They silenced the isles, broke his hold on Thergon's dead, and drove him off his own throne and out of the Western Isles entirely. He went east, and what he built there is Chaal Nazzerox.
What remains is a city held in the moment the Silencing fell on it. The coral halls still stand, salt-stained and open to the sky in places, and the dead still stand in them, the ones Xynoth could not carry with him and no longer commands. They do not rot the way a corpse should, because a held shadow keeps the body it is knotted to. They keep to the streets and the temple steps and the waterfront, moving on the last orders they were ever given, which they will follow until something cuts them down. Nothing has cut most of them down. There are a great many of them, and the city makes no sound.
The throne of Thergon, cut from a single mass of blood-red reef coral and never moved. Xynoth Azkonor was crowned on it, died on it, and held court from it as a lich, and he left it behind when he fled. It sits in the open hall with the sea coming through the broken wall at its back, unburned and unlooted. No one has carried it off. It weighs more than any boat will hold, and besides, no one wants it.