The History of Bastion's Keep is a tome in Act III of Diablo III. It can be found on the Librarian's Scrolls in The Keep Depths, as well as The Underbridge. All parts are need for the Historian of Arreat achievement.
It reads as follows:
"Many centuries ago, King Korsikk built Bastion's Keep to "pen the barbarian threat" in the north. After years of military disasters, the frustrated king led a large army out of the fortress and into the north to conquer the barbarians once and for all. Not one of them came back."