It's based off a series of novels by Mercedes Lackey. Many of the novels focus on the Kingdom of Valdemar, so that's become a shorthand reference to the entire series/world.

http://en.wikipedia.org/wiki/Valdemar%28fictionalcountry%29 (will redirect to "Velgarth", which according to one in-canon reference is the name of the planet)

There's assorted links to articles on the novels at http://en.wikipedia.org/wiki/MercedesLackey#Valdemaruniverse

All we've got on the PPC Wiki is a little stub, at least that's all I'm finding.

There's also some information at her personal site: http://www.mercedeslackey.com/