Dawnguard, an expansion for The Elder Scrolls V: Skyrim, is now available on the PC.
From Bethesda Softworks
The Vampire Lord Harkon has returned to power. Using the Elder Scrolls, he seeks to do the unthinkable - to end the sun itself. Will you join the ancient order of the Dawnguard and stop him, or will you yourself become a Vampire Lord? Featuring an all new faction questline, new locations, weapons, creatures and more; the ultimate choice will be yours.