In centuries past there was a large Celtic Empire that spanned thousands of miles. Reaching from Gbor Nor in the East and extending Westward over the Great Mountains into what would become Terraguard and along the northern sections of the Great Trade Route. Wars were fought over land that eventually forced the Empire back to the eastern side of the Great Mountains. From the Northern Wastes and stretching south to where Fuirgar became another point where the Empire's growth ground to a halt. The empire fell during a short period of internal wars fought between mages and priests. This was precipitated by the establishment of Wolfspack on the western side of the Great Mountains as an independent city in 371 CER.
One of the effects of the fall of empire was that the Celtic gods and their worshipers have been split into two versions. There are druids (with a small d) that are priests of the various gods worshiped by the many believers in the Celtic World and the many believers on both sides of the Great Mountains. Then there are the Druids (capital D) that hold to old values, such as human sacrifice; and get their powers from the pantheon overall rather than an individual goddess or god. The Druids are found throughout the Celtic World, and particularly strong in Hallstatt.
There is still a large presence in Terraguard of Celtic culture primarily in the southeastern portion and some of the Great Trade Route. The Celtic World is the modern remainder of the Celtic Empire and where Celtic culture is strongest.