Bethesda's first official add-on for The Elder Scrolls V: Skyrim, called Dawnguard, is available now for Xbox 360.
The DLC introduces new antagonist Harkon; a Volkihar vampire lord hell-bent on destroying the sun and ruling the lands of Skyrim. The player can choose to aid Harkon on his quest (which opens a new set of vampire-based skills/perks) or join the Dawnguard's crusade against the bloodsuckers (essentially turning you into a medieval version of Buffy the Vampire Slayer.)
The Dawnguard DLC also ushers in a host of improvements to the main game, including access to crossbows, the aforementioned vampire perks and mounted combat.
Skyrim: Dawnguard is available now and costs 1600 MS Points (around $25). The add-on is expected to hit PC and PlayStation 3 at a later date. Check out the full trailer here.