Dracul is the unholy deity of death, undeath, evil... your archetypical Lord of Darkness. His boons are well suited to brawler-type characters, and with only a few very narrow exceptions is largely useless for spellcasters or striker-type attackers such as the rogue.
Dracul is unlocked in a subdungeon to the east. When you enter this subdungeon, you will be pitted in combat against a doppelganger of yourself. However, there is a known bug where the subdungeon exit doesn't seal itself off until you move one tile. If you adjust the visible tabs, you can leave the subdungeon and avoid unlocking Dracul in this fashion.
Dracul is difficult to gain piety with under most circumstances. There are only three ways to do so: killing non-undead monsters, converting a "holy" glyph ( CYDSTEPP or HALPMEH), and using a life-steal ability. For most characters, the only way to get a life-steal ability is with one of Dracul's boons, which means it's imperative for Dracul worshippers to purchase either blood hunger or blood tithe as soon as possible to begin racking up real piety.
Dracul's boons all grant ways to restore your hit points or (in the case of blood shield) prevent incoming damage. This gives a Dracul follower greatly increased staying power in combat, and he often makes an excellent last-ditch choice in the end-game since none of these are dependent on exploration space.
Both the blood tithe and the blood hunger boons provide a balance of piety gain and healing. They both have serious drawbacks that must be managed, but with a large number of weaker monsters to "farm" for lifesteal healing they can allow a significant boost to your end-game healing prospects. For characters who can raise their resistances above 45%, you should virtually never purchase the blood hunger boon. Blood Tithe can offer some reasonable healing if you select it multiple times, but you'll need to find a way to raise your maximum HP to compensate and it will cost a significant amount of piety. This usually means you'll need to convert into Dracul's religion with a large amount of piety already accrued, and found some way to increase your hit points.
The need for improved hit points means Jehora Jeheyu, The Earthmother or Glowing Guardian all make excellent deities to worship prior to converting to Dracul. Followers of the Earthmother should take note that her "plantation" boon will remove bloodstains. Followers of the Glowing Guardian tread a double-edge; absolution will improve your maximum hit points and improve the effectiveness of sanguine, but it removes monsters (and therefor blood pools) from play and therefor limits your ability to use sanguine. Normally removing somewhere between 10-20 monsters with this boon is ideal, giving you a balance of additional hit points without leaving you without potential targets. Jehora Jeheyu is difficult to manage, as most players will want to use Chaos Avatar before converting out, which may leave you with insufficient piety, but this combo is very powerful when you do have enough piety to pull it off.
When combined with the BLUDTUPOWA glyph, Dracul becomes an awesome force for spellcasters. His blood tithe and blood hunger boons can be used to farm hit points, and then BLUDTUPOWA can be used to convert them into mana. This allows Dracul to fuel some of the most powerful spellcasting spikes in the game, turning any character into a Bloodmage.