This trophy is for unlocking all 11 Vampire Lord perks. The Vampire Lord is a special transformation unlocked through the main Dawnguard DLC questline. You unlock perks from killing enemies with your
Life Absorb ability in your floating mode or by holding
near a humanoid NPC who's near death, in your grounded mode. The former is much easier to kill with so go with that.
For starters, I'd recommend both making a manual save, lower the difficulty to Novice, and removing Serena as a companion as she weakens the power of your Life Absorb ability for some reason. Now that you're all set, you can start to fast travel from each of the major cities, killing anything and everything in your path with the Life Absorb ability. On Novice this should be fairly easy and most things will only take 1-2 to kill. Make sure to go outside the cities and kill the enemies out there too. Once you've done this with every city, you shouldn't have too many perks left. I started travelling the roads at that points killing wild animals, bandits, and travelling NPCs.
Using this method, you should be able to get all the perks and the trophy in around an hour and a half or less.