Another consideration, if it applies, is whether it stacks with other ATK buffs. Otherwise, it's something to DoT yourself with if you're going to fight a mezzing mob, a la Ireblind Imp, or BRD/ENC.
The 4hp/tick on my Iksar SK is pretty much nothing as he regens faster than the 4hp.
Even soloing, an SK can siphon HP readily to make up for the HP lost - and in a group that really is nothing to a cleric that complete (or celesital) heals you.
But - the added attack is definately a plus and well worth it. All in all its a great item to get and relatively cheap. A 30s SK should be able to get the spell with a few LDONs.
This spell is a great tool for an sk....atk bonus is always a plus...20 atk= vengence 4 come on that rox!! not to mention the low damage dot is a enchanters/bards nightmare come pvp.... just try to mezz me i dare ya....pwn'age
Maybe I'm not seeing the big picture, but even at level 34, is 25 ATK that big of a deal to sacrifice 4 HP/tick? By level 34, you can tap for 8, and buff for 10. Five levels later, at 39, we get an ATK tap for 20 - that's 20 for us, and 20 from the opponent. This "Temptation" line give a sizeable ATK boost, but in effect, DoT's yourself. My issue with this is: is 25 ATK enough of a boost to justify losing 4 HP a tick?
I do not personally play Shadowknights, thus I am not too familiar with the spells and effects you have mentioned, so I am assuming they are correct.
25 ATK at a cost of 4HP a tick on its own is bad, especially if you solo a lot, but it would be good when you are in groups. Seeing as this spell is bought in LDON shops, it might have been designed with that purpose in mind. Slap on a good regen, and the DOT is pretty much nullified.
Also, it is useful to have as much ATK as possible in LDON missions, given the fact that the missions run with a limited timer. Higher ATK = faster mission victories. Time is not a luxury in LDON.
In just about any group situation, you're going to have a healer, which basically makes the VERY minor DoT a non-factor... Also, check out the duration -- Compared to Grim Aura, this is a godsend.