The theorycrafting on this potent weapon is still rather sparse, so I spent several hours testing the limits of the proc and it's dps contribution. First off, the basics:
The 264 version proc has an average damage of 2250, while the 277 version has an average of 2538.
The damage does not scale with any character stats (though the dps does, more on that below).
The damage does scale with any percentage damage increases that affect spell damage, such as Avenging Wrath, ***** of Elements, Blood Presence, etc.
The drain can proc off of any weapon attack, as well as some odd events such as applying or refreshing effects such as deathknight diseases or the paladin talent Righteous Vengeance. These odd events do have some requirements though. The drain can proc off of the refreshing of some DoTs or debuffs so long as the character is within melee range of and facing the target for the drain. Specific to the deathknight class, the drain can proc off of the disease application of Icy Touch (assuming you are in melee range), can double-proc off Plague Strike (the strike and the disease application), and on each target that is in melee range and in front of you from Pestilence.
The drain has a base proc rate of 2 procs-per-minute, or 11.3333% chance per swing, and has no internal cooldown.
The drain can proc multiple times per attack, provided there's are more than one weapon event for the attack. For example, the drain can multi-proc off the following:
- On any or all of the multiple targets of Cleave, Whirlwind, Divine Storm, Heart Strike, or Sweeping Strikes.
- From a melee (or special) and it's subsequent seal proc, Blood-Caked Blade proc, or Windfury proc.
- From both the physical and shadow portions of Scourge Strike.
The drain can not proc from any spells, excluding the odd events mentioned above.
The drain is incapable of critting and functions on the spell hit table. However, it doesn't seem to be resistible.
Now, with a 2 PPM baseline, the weapon will deal 2 procs a minute before any waste or special attacks or talents are factored in, giving it a minimum dps of 75. In practice, the proc will usually provide between 200 and 400 dps, depending on the character's haste rating, spec and rotation (and therefore number of procable attacks per second), spell hit chance, and +damage buffs present. A good estimate is that it will provide approximately 4-8% of a character's dps. For many classes, this means the proc is worth the equivalent of between 400 and 800 additional attack power, though it's place on the best-in-slot list is strongly dependent class, spec, haste rating, and rotation. In general is can be assumed to be near the top of the list, however, as the proc itself provides a fairly respectable amount of dps while also making it easier for the healers to keep you alive through miscellaneous raid damage.
