Mash together the concepts behind the Painted World of Ariamis and the winter wonderland of Frozen Eleum Loyce, and you've got Ashes of Ariandel, the first of two planned expansion packs for Dark Souls 3.
As with Dark Souls' Artorias of the Abyss DLC and Dark Souls 2's Lost Crowns trilogy of expansions, the Ashes of Ariandel add-on has been grafted onto pre-existing content. This means you should be able to find it if you've explored at least halfway through Dark Souls 3's base content; you just have to know where to look.
Prepare to Die
Fair warning: FromSoftware's devilish designers tailored Ashes of Ariandel toward high-level characters. It's recommended that you wait to try your hand at the new area until you get a character somewhere between levels 60 and 70 at a minimum. Better yet, finish the game first to stock up on Titanite chunks and slabs. Go in with your best weapons equipped and your most damaging spells attuned.
Ashes of Ariandel takes places inside the painted world of Ariandel, a snowy landscape dotted with soaring towers and fortresses hidden behind a curtain of stinging snow. Before you can set foot in this new domain, you need to locate a new NPC in the Cathedral of the Deep.
You'll find him by warping to the Cleansing Church bonfire. He's kneeling beside the altar between the church's two sets of large doors. Exhaust his dialogue and you'll trigger a cutscene in which he hands you a painting... or rather, a scrap from a painting. Magical shenanigans occur and you're spirited off to the frozen wasteland of Ariandel.
Your mission is to discover what ruined the once-fair domain. Hint: the answer probably has something to do with the big guys wearing crow masks and slicing you to ribbons with their Wolverine-like claws.