[catabur]

Right now, I don't think Black Drain is really that great. 

It's -10 SWA for the enemy, +10 SWA for you, but it relies on you having to have used Black Wind and then using a base Soldier skill. However, this is all at max level and unless you're abusing Thousand Stabs then you'll only slowly crawl your way up there. It's 6M, around 30 FP, and depending on your weapon it could take you multiple turns to get the full effect of it. 

I would propose increasing the max rank to 20. With Black Drain LV 20, you would be countering Two-Handed which fits the defensive nature of Black Knight quite well. If a 20 SWA increase for BK seems too much then it would be simple to change how much SWA you get from it: Black Drain LV/2. 

Furthermore, increase the Black Drain LV you get from a singular attack to 3. Alternatively, make it so Basic Attacks also count as giving Black Drain. 

So, the Skill ranks would look like:
1 Point: 7
2 Points: 14
3 Points: 20

[Poruku]

I feel like keeping it at max 10 but raising it to 2 per hit would already be quite good

[Rendar]

A 20 SWA shift would be nutty. 10 shift is already pretty big, but having it actually scale at 2 per would be ideal.

[Senna]

T/Stab with an element badge. Bash after, that gives you 9 in 1 turn. If your gonna do the +2 per thing, gotta do it with that in mind.

[Maromar158]

What if we made Black Drain an active ability that becomes available after invoking the wind? Just grab someone's face like an anime villain and slurp all or half of the 10 SWA and get a little health, too as a treat?

[Autumn]

Because it's both adding to your's and subtracting from their's, 15 at max sounds a bit more reasonable, but its as you say, unless you're abusing thousand stab, it doesn't really stack fast enough to be all that worth it.

[Poruku]

Yeah or low hp mad chop, but well, at that point its less useful. It would be better if it was something like, gain 5lvl on skill hit, not per instance of damage. Would make it quite good with things that have an aoe