Ashes of Ariandel is the first of two planned DLCs for Dark Souls III; it adds a new region to the game with a new boss, new enemies, new armor sets, new weapons and spells, and a PVP arena that can be accessed from the Firelink bonfire once you find and beat the second, hidden boss. For $15, it'll get you about four hours of content and at least one new toy for each type of build, which you can put to use in the arenas for 1vs1 duels (un-embered, no estus), free-for-all brawls (timed match with respawn, limited estus, player with most kills wins), or team-brawls (same as free-for-all, except 2vs2 or 3vs3). For the most part, it's all quality content with memorable encounters and fun new weapons, and the PVP arena will really help extend the game's life for those interested in PVP.
Despite its overall quality, Ashes of Ariandel wasn't that satisfying for me. Part of that has to do with its relatively short length; I was able to explore everywhere and do everything possible in a single afternoon, and the whole thing felt anticlimactic. In typical Souls fashion, the story is practically non-existent, with you entering the Painted World of Ariandel on an incredibly vague pretense, and then wandering around aimlessly until you trigger its ending, which leaves everything almost completely unresolved. In the end, this DLC felt more like it was a hidden, optional area that was cut from the base game instead of a proper DLC expansion. It's not a bad experience, mind you, but apart from the PVP arena I feel like I wouldn't have missed much if I'd just skipped it altogether.