"A World of Magic"
Tracing its roots to GURPS Fantasy First and Second editions, GURPS Banestorm details the land of Yrth, expanding considerably on what was published before.
From the back cover:
"Welcome to the land of Yrth, a realm of incredibly varied races and monsters – including people snatched from out Earth and other worlds by the cataclysmic Banestorm! Now humans struggle with dwarves, elves, and each other. The Crusades aren't ancient history here – they're current events. And on this world, magic works!"