The Pity Knights were an order of lich-hunters, and they were the best the world ever produced at it, which is a grim distinction to hold. A lich cheats death by splitting its own spirit and wearing a soul and shadow stolen off the living. To find one, corner one, and end it for good, the Knights had to understand that theft in exact detail, and so they studied the working from the inside. They used Deoric themselves, the same life-spending command-tongue that makes a lich possible in the first place. They used it well enough that for generations they were the one thing a lich had real reason to fear.
What pity meant
The name was not irony, though it reads that way now. The order held that a lich is a soul in flight from its own death, fragmenting a little further with every escape until nothing recognizable is left, and that the monster is only what the flight leaves behind. To hunt one was an act of mercy before it was an act of war. The Knights granted the true death a lich had spent everything to refuse, and they held that this was a kindness even when the lich fought them over it. Pity was the discipline, not the sentiment. A Knight who came to hate the thing he hunted was a Knight who would begin to enjoy the work, and the order did not keep those long.
We do not kill the dead because we hate them. We kill them because someone must love them enough to let them go. — the creed of the Pity Knights
Thergon and the fall
The order ended at Thergon, and it ended because of one of its own. Xynoth Azkonor was a sworn Pity Knight and the king of Thergon, a coral city in the Whisper Isles, and when he died he used everything the order had taught him about how a lich is made to keep from staying dead. He split his spirit, came back, and took his own kingdom by force, raising Thergon's dead into the first army he ever commanded. The Knights had trained their whole lives to hunt exactly this. Now it wore the face of a brother and a king.
They came for him in strength. Maradoth Selvath, the Pity-Marshal who led the campaign, found what no Knight before her had been forced to face: Xynoth had split his spirit too many times to be ended by any working they could land on him. He could not be killed. So Selvath made the choice the order is remembered for, and it was a terrible one. If they could not kill the lich, they would take the city out from under him. The Knights spoke a Silencing over the whole archipelago, a prohibition wide enough to forbid the command-tongue itself to carry, and it broke Xynoth's hold on Thergon's dead long enough to drive him off his throne and out of the Western Isles. He fled east, and what he built there is Chaal Nazzerox, larger and worse than anything he had held in the isles.
The campaign that saved the Western Isles destroyed the order that fought it. The Knights had spent their strength silencing a kingdom and still left their quarry alive to flee and build a worse one. Worse than the failure was what Xynoth proved. The only thing dividing the hunter from the prey had ever been the oath, and the oath could be broken from the inside, by the most accomplished Knight among them. After Thergon the order did not recover. The survivors disbanded within a generation, and no one has taken up the name since. The Silencing they spoke still holds. The order that spoke it is gone.