Note: Please excuse any weirdness in this guide. I am running on 2 hours of sleep today.
Damage - He does one thing and he does it really well
Secondary Tank - If your front liners go down he's a pretty good speedbump to shield the rest of your party (note: this role becomes more prominent at higher levels)
Dragon - He can fight in Dragon Deathmatch and should stay away from Dragon Slayer
Physical basic attack, magical skills
Revenant - This is Dark's big damage dealer and biggest use. To break it down, it deals damage over time for 10 seconds (a stupidly long time) to all enemies and heals himself for a portion of the damage done. Now the big thing about this is that the heal is independent of Magic Vamp, meaning he can get healed twice....per hit...per enemy....for 10 seconds. This is insane. Oh and don't forget he's causing damage the whole time. A fantastic skill.
Impale - This is Dark's "nuke" attack in regards to the fact that it does all of its damage upfront. It hits everything in front of him, which is "usually" the whole enemy party. This can get screwed if he gets turned around by a Skeledeer or Ninja Dwarf. Good damage, quick animation, nice AoE, definitely worth your time.
Dark Shadow This is a very interesting passive. This gives Dark Dracul innate Magic Vamp, which heals him whenever he does magic damage. Magic Vamp (unlike Life Steal) is not easy to get on a character. In fact, only Dracul, Spirit Wolf, and Water Wyrm get an ability that provides it. This is a great skill, which boosts the healing from Revenant and heals when he does Impale. Giving him great survivability.
Cloak of Night - Just when you thought he couldn't get tougher, he comes cloak of night. This unique passive gives him extra Skill Power (increasing the strength of his magic) and increases his max health based on his total Skill Power. In my experience, this usually tacks on an additional 50% to his max HP.
Dark Affliction - This is a special Legendary Skill that is great on paper, but has some issues with Dracul's skillset. This ability deals extra damage based on how much HP the target has lost. This bonus adds up VERY quickly and can cause some big hits. The issue being that Dracul's biggest Damage contribution is a Damage over time skill and Dark Affliction does not recalculate the boost between each click. Still a great damage boost and worth going through the quest to get.
Ocean is the big one to me, more skill power buffs all of his attacks as well as give him extra health due to Cloak of Night. I can see where someone might want to use River, but honestly I have never had an energy problem with this character. For the remaining two slots it is entirely up to you to pick between Tree (Even MORE HP), Rock (Patch up the lower of his two defenses) or mist (turn him into an anti-magic wall). All three of those work fine on him.
Hydra - You know what's even better than his Revenant skill? Revenant with a HUGE power boost. Hydra's Legendary buffs Revenant's power and the two dragons form a great core. Both are physical/magical mixes but Hydra's DoT is Physical and Dracul's is Magical meaning they can corrode any target between the two of them.
Groovy Druid - An odd pairing but one that works fairly well. Druid's ability to break shields is handy, as is her reduction of enemy Magical Defense.
Plant Soul - You know what's better than massive HP regeneration? Even MORE HP regeneration. If Dracul dies with this pairing you are fighting something WAY above your level.
Spirit Wolf - Another magic vamp hero, but this one can turn Dracul into a genuine tank with Feral Taunt (since Dracul is a strong contender for being the party member with the most remaining HP), also a crit boost is always welcome.
Raging Revenant - This is a one-sided pairing since Dracul doesn't base his damage off basic attacks, but he does supply Revenant with a great source of HP for Reckless Remedy.
Shadow Assassin - Her Assassin's Mark boosts the damage of Impale and that's about it
Dragon Slayer - He's a dragon, she's a slayer. Enough Said
Orc Monk - An uncommon hero, but a strong counter. Virtue of Necessity can cut Dracul's damage in half, and Righteous shield can clear away Revenant and provide a shield against it.
Ninja Dwarf - Dracul has high intellect and Ninja will hunt him down. This matchup is dependent on levels mostly, whoever is higher will usually win.
Cosmic Elf - Like all spellcasters being silenced is a bad deal for him.
Banshee / Wee Witch - Both of these heroes have the ability to turn the heals from Revenant into damage, very dangerous since Dracul can't shut off revenant once it begins.
Sun Seeker - A lesser version of Orc Monk, Heaven Rays will reduce necro damage, but not as well as Monk will.
(+) A very powerful damage dealer that is easy to improve
(+) Highly durable with a near constant stream of HP coming back to him
(+) Skills hit a wide area increasing his damage output
(-) Damage over time is high, but hard to notice. Unless you check the damage at the end of combat it may not seem like he's doing much
(-) He and Spectral Drake have a hate/hate relationship. Whichever one gets off their white skill first is gonna win
(-) A very selfish hero, none of his skills improve anyone else or heal other party members