C. S. Friedman’s 1990 The Madness Season is a standalone science-fantasy novel.
Centuries ago the Tyr had crushed the Earth. The Tyr are many bodies but one immortal mind. Consequently, they do not truly comprehend humans. They demand obedience from their peons. Immediate execution is the usual punishment for any deviation.
Daetrin Haal is eager to keep a low profile. He has a secret, which is that he’s immortal. As such, he’s a threat to the Tyr. He remembers the pre-invasion human past, which the invaders have carefully erased.
His mind and his memories make him a threat to the Tyr order. Eventually he is noticed.