This is a storyline achievement and is unmissable.
This is awarded for the final quest in the dawnguard storyline and unlocks after the boss battle with Lord Harkon, which occurs despite who you side with (vampire/dawnguard).
If you sided with the dawnguard as I did, you will assault the castle with them and fight through vampires and gargoyles to reach the castles chapel. After the conversation Harkon will attack you with the aid of skeletons and gargoyles.When you get his health down he will retreat and heal behind a shield, which can be broken by using Auriel's Bow and Sunhallowed arrows. When he dies, the achievement will pop.
Note that I didn't break the shields due to being in werewolf form and after regenerating three times he finally died. Auriel's bow is probably recommended to shorten the fight.