Currently in Classic, Improved Drain Soul triggers if I or my pet kill ANY mob with ANY ability while channeling Drain Soul on ANY mob, including a mob other than the one that was actually killed.
That’s right, Mob A
can die to my dots while I’m draining Mob B
's soul, and the improved drain soul effect (Soul Siphon) will still trigger.
In Vanilla (up to and including patch 1.12), it would only trigger if damage from Drain Soul itself was the killing blow. It had to be a Drain Soul tick. No other damage could trigger it.
In patch 2.0, it was changed to trigger if the warlock landed the killing blow on a target while draining its soul.
In patch 2.1, it was changed to trigger even if the pet landed the killing blow.
Now apparently in Classic, you merely have to land a killing blow on SOMETHING while draining soul, even if the thing you killed is not what you’re channeling drain soul on.
I’m pretty sure the talent is still bad, but it might be worth it now. LOL