The White March - Part 1 is the first of at least two planned DLC-sized expansions for Pillars of Eternity, Obsidian Entertainment's recent isometric RPG. See my review of the base game here (short version, I liked it a lot). This expansion introduces a new region of the world map with several explorable areas, a bunch of new quests, new enemies, new types of equipment, and a couple new companions. The new content takes place roughly in the middle of the main campaign, once you've begun the second of three acts and have gained access to the player stronghold, Caed Nua, but before moving into the game's final dungeon.
It certainly makes sense for Obsidian to put the DLC in the middle of the game -- it wouldn't really work if it took place after the base game, since the ending specifically talks about your companions going their separate ways and describing their lives in the months that follow your adventure -- but it's a bit of a nuisance in practice. For anyone who's already completed the game, that means you have to load a save from before the "point of no return" and sort of pretend like you didn't already beat the game, and for anyone playing for the first time, the DLC will feel like a huge deviation from the careful narrative pacing and mechanical balancing of the base game.