Horseclans was a science fiction series by Robert Adams, set in a North America that had been thrown back to a medieval level by a nuclear war.

The book includes the history and background of the future world and the land of Mehrikah, plus rules for creation of Horseclans characters, numerous psionic abilities, and mass combat.

[1] Horseclans is set about a thousand years in a post-apocalyptic future after the collapse of our present civilization (apparently by a nuclear war).

They are opposed mainly by a small group of Undying, individuals who don't age and are very hard to kill.

[1] GURPS Horseclans was one of the earliest licensed properties produced by Steve Jackson Games.