The Highlands of Avalon DLC features a brand new island type (the Highlands) with new tile types, items, locations, enemies, recruitable characters and more. Once the DLC is activated it will expand possibilities across the entire game, both in Campaign Mode and Director Mode.
Full feature list:
New island type: Highlands, with a completely new set of tiles, a new game mechanic related to traversing hills and valleys, a new tribe called the Picts and a new artwork for shrines