Some brief things about Shadowfiend, taken from Elitist Jerks:
- Attacks are melee shadow damage, using a normal melee attack table, with damage reduced based on armor of the target
- Has ~1.5second attack speed (10 times per 15 second life)
- Benefits from your +dmg with a ~65% coefficient (6.5% per hit)
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Damage added from Shadowform, Darkness, Shadow Weaving, Misery, ***** of Shadow, and Improved Shadow Bolt, and so on.
- Damage added from attack power bonuses (possibly double-dipping - the direct buff, and scaled with your stats), armor reductions on the target, and so on
- Benefits from Bloodlust/Heroism and similar buffs IF the fiend is already out when it's cast.
- Has extremely high dodge rating (in the vicinity of 75%) and significant resistances to most/all spell schools