The Bladelands Chronicles

Volume 1, Chapter 4:

A number of Keltoi wise ones aproach a boggy area to the North of Fort Winnofar, in an attempt to perform a ritual to subdue a source of undead there, but are disrupted in their task by residents of the Fort. As a result a large number of undead appear and terrorise the locals, until the area is cleansed and the portal through which they are appearing is closed.