Something I've noticed is that Shadow Maul does the same. And, with it's very high defiance rating, if you hit 3 or more enemies with it, you're basically getting a build-up from that alone.
Main Hero: Mazey - level 50 + 1 fire/fire/fire blaster.
Main Villain: Chained Bot - level 50 + 1 Robot/FF Mastermind.
BattleEngine - "And the prize for the most level headed response ever goes to Mazey"
The way I'd played, Shadow Maul stopped being useful in the mid-30s. Even with its defiance boosting power, the animation time is so long, and its damage is so low, that I was better off cranking up my recharge via IOs and subbing an ST Blast in to throw together a Melee chain rather than punchapunchapunchapunchapunchapunchapuncha. Especially with Soul Drain on a shorter recharge time, I didn't really feel the need to do anything but use my fast-animating attacks while they could one-shot minions and some Lieutenants.
I'm on a Dark/Dark/Mace Blaster, though. I bounce in and out of range as needed, trusting Scorpion Shield, IO defense bonuses and my ToHit debuffs to keep my Blaster alive.
Too many alts to list.
Curious if this is WAI or a bug or what, but I've noticed that when I use Soul Drain I'm getting not only the base 50% boost and 10% boost per enemy, but also a defiance boost (6.3%) per enemy hit, which I haven't seen from other aoe powers. 3 enemies is putting me at 98.9% (50 + (3x10) + (3x6.3)) instead of 86.3 (50 + (3x10) + (1x6.3)) as I would assume with just a single defiance boost.
Am I misunderstanding how defiance works in this case, or did I just get kicked out of Dark Manipulation Fight Club?