Shadowfiend appears to be classified as shadow melee damage. It benefits from your +dmg at a 65% ratio (10 hits at 6.5% of your +dmg each), and the damage is boosted by Shadowform, Shadow Weaving, Curse of Shadows, and Misery (and possibly Darkness?). In addition, because it's melee damage, it also benefits from any AP buffs (and double-dips them... gets strength from Strength of Earth totem, and benefits from the extra strength you yourself have), armor reductions on the target, and so on. It also has a standard melee hit table (crit/hit/miss/dodge/parry/glance, not sure about block), and as such benefits from being dropped behind the target (in order to negate parry chance). Finally, it benefits from Bloodlust, but only if Bloodlust is cast after it is (so the buff is actually placed on it).
He was asking 6% of 9000 because fiend costs 6% of your base mana. However, this is BASE mana (when you're naked), meaning it's cheaper than a talented Mind Flay... I want to say either 187 or 157 mana (I seem to have dyslexia with those two numbers), but I may be off a bit.