From the story and from their own abilities. You can see that the Dark Knight's theme is Justice and honestly.. Pain. Their inner darkness has to be fueled from something and it seems like they hurt themselves just to grow stronger and find their inner Abyss.
I am not going to try to change the entire class like I usually try to. However honestly the Dark Knight is way too much like Paladin. Nobody can deny that and they seem a bit rushed in their design. Here are some of the ideas I propose.
Yoshida: That concept of sacrificing in your HP in order to raise your attack power we’re still considering whether or not to include that sort of action in the Dark Knight skill set. Of course since it is a tank role the Dark Knight you don’t want to go out there sacrificing your HP to raise your damage! No sacrifice!
Indeed, I agree. It is silly for a Tank to kill himself for DPS. However, i've been thinking. How about killing yourself for powerful defense? For example.
SentinelShadow Wall
Reduces damage taken by X%.
Sacrifice X% (same amount as damage taken.) of your max life to cast.
Duration: Xs (I won't put a duration there.)
Recast: 5s (Its meant to be up when you need it always. However it costs your life to cast it. So its not something you spam!)
You have a very nice thing now. Your sacrificing your life to lower the damage of attacks you receive. You could be wasting your life. However you could also be reducing much MUCH more damage then what you took to cast it. Since the cool-down is low as well it would be great to use in emergencies when you need to deflect a big hit. It also fits the "sacrifice" theme pretty well.
Abyssal Drain
This has always bugged me, I like the ability and want to cast it a lot. However, I find the cheaper to use Unleash better for me. It hits targets regardless if i'm facing them and even though the animation is atrocious I find that it seems to raise enmity more.
The Dark Arts healing does seem cool, however for costing the Mana it does plus casting a Dark Arts just to make it heal.. it doesn't really seem worth it honestly. I could see situations for when it does but I think I just like using Unleash because its more cost efficient and if I'm doing lower level content I don't have to change my toolbars.
Deals unaspected damage with a potency of 250 to target and enemies near it.
Additional Effect: Absorbs 200% of damage dealt as HP for the first enemy, 20% less for the second, 40% less for the third, 80% less for the fourth, 60% less for the fifth, and 100% less for all remaining enemies.
Can only be used when under 50% Life.
Recast: 40s and now instant off-GCD.
STILL COSTS MANA!
This change still makes it a cool ability but now it goes more with the theme.
These two changes, I think would make the Dark Knight a lot more fun to play with their own bits of mechanics.
It is not "Poor Design" its poor usage. You use it when you know your going to be taking a lot of damage quickly. You use it when you know your healers might be overhealing you, which you can plainly see if you look at them casting.
Its very good design actually. It just takes a very high skilled player to see it. Its worth the sacrifice in most cases.
If you just spam it, its poor design. If you use it properly, its amazing.
ITS A VERY GOOD CD TO USE AGAINST TANK BUSTERS!
Lets say you have 15k health, and you are facing a 20k tank buster. They usually have a long cast time so you see it.
You use your CDs, reduce the damage with Shadow wall, then get healed up or topped off, then reduce the damage more with your tank stance. NOW you took way less damage then you sacrificed.
Its very good if you know when and how to use it.
I play a healer and I play a tank. I know for a fact if you are at full life and suddenly lose the health I would have a bigger heal planned for you already. Healers a lot of the time actually overheal you, however this reduces your out going damage, which is really good because it prevents a lot of damage you could take. Which can be double or triple or quadroople what you paid for.
*Example* Sacrifice 2500 health to possibly mitigate 10000 damage. It is a very good design. Just need to use it properly.
If you just spam the ability or CDs every time they are up. I can imagine them being bad. However you use the ability whenever you know your going to take a lot of damage or you know your going to be overhealed. Its "SKILL."