The Dawnguard questline allows you, the dragonborn, to become the most powerful vampire ever known. You can choose this path once you free Serana and return her to her castle, where you will meet Lord Harkon and be offered his blood. Once you accept you are then given multiple missions which eventually lead to you to begin searching for a legendary weapon, known as Auriel's Bow. Auriel, is one of the eight (or nine) divines, so naturally his bow will carry great power. Once you slay countless evil creatures and reach the bow, you must take it back to Harkon and challenge him with it, because in the process of finding the bow you realize you don't really like him. It ends with you slaying him, assuming your rightful position as Lord of the court (of vampires), and may use the bow to cloud the sun, making you and other vampires immune to its mal-effects.

These are the four stages of hunger, and it's effects on the player. The first is the most mild, and four is blood-starved. Keep in mind, that the hungrier you are the more powerful you are, but the weaker you are to fire and sunlight.
One	Two	Three	Four
This is the stage you achieve after feeding. It prevents the most damage from the sun and from fire, making you only 20% weaker to fire damage and reducing your health, magicka and stamina by 25 points each while in sunlight.	The second stage, reached after going one day without drinking blood. Not the end of the world but still inconvenient as it makes you 40% weaker to fire damage, and reduces health magicka and stamina by 40 points each in sunlight.	Now becoming more of a problem, this stage is gotten to after two days without a taste of blood. It reduces your fire resistance to 40% its normal amount, cutting 60% off of your normal resistance. It also shaves a decent 65 points of health, magicka and stamina when you stand in sunlight.	Ah, you are now officially starved. It might not be comfortable, as it makes you resistant to 80% less fire damage. That, and it also cuts down your health, magicka and stamina by a whopping 95 points each while in sunlight. Maybe sunscreen could help?
You are at your weakest, with little vampiric ability and a lifesteal spell that only takes five points of health for every second you cast it.	Gaining power, you now have more vampiric ability, though less charming you are more deadly with a lifesteal that now takes 12 points of health for every second its cast.	Approaching your full potential, you now have even less charm, and a more thirsty, evil look in your eyes. With a lifestealing spell that now takes 25 points a second, you're sure to take down any creature in your path.	You are now at full power. The citizens of Skyrim now fear you, as your true form is now clear to all. But they don't know half of it. With a lifestealing spell that takes a massive 40 points a second, you're sure to leave no witnesses to your power.

Brief Summary