So, for most of us, we have probably come to realize that tanks in PvP aren't meant to hold aggro so much as they are meant to reduce damage, protect allies, and harry enemy healers and ranged damage dealers.
Now, while I find Dark Knight's damage output fine, they do more than enough to trouble healers and their abilities are adequate at reducing damage and protecting allies, they have some crippling holes in their abilities, most of which are simply there from a lack of foresight or planning. Dark Knights are hard to kill, very hard to kill, which is a good job done by the developers, but that is not what a tank is in PvP, and no matter how hard we are to kill, no healer can keep up with a tank that is taking 250% extra damage should it come down to that. So let's go over fixing up the four abilities that have the biggest issues in PvP, and what, in my experience, could be done to solve the problems these abilities have currently.
Carnal Chill is probably the poster child for abilities that are much weaker than they should be for awkward reasons. Dark Knight is an interesting case as a tank in PvP, since while Paladin and Warrior have highly defensive abilities for party damage reduction in the forms of Testudo and Thrill of War, Carnal Chill is highly offensive. Not only is Carnal Chill offensive, it is a very large burst of defensive power, but lasts a very short amount of time compared to its peers. In many ways it is a harder to use version of Testudo that deals damage. But unlike Testudo and Thrill of War, Carnal Chill doesn't effect adrenaline rushes, the highest burst abilities in the Feast due to how adrenaline rushes and limit breaks work. This makes Carnal Chill feel rather weak, since while yes, nerfing a damage dealer's output by 60% is a huge change, the servers' netcoding combined with how adrenaline rushes and limit breaks work, leaves Carnal Chill feeling weak when it should be stronger. The most reasonable change would be to let flat damage reduction from abilities like Carnal Chill and Storm's Path apply to adrenaline rushes, as well as maybe giving a grace period to debuff applying since those abilities cast so very quickly.
Moving on, we have Dark Knight's and Paladin's arguably, most powerful debuff, Blind. Both Dark Passenger and Flash are incredibly powerful, but in PvP, like PvE, enough accuracy defeats the point of being blinded by enemies. This is a fact most of us already knew, but it seemed worthwhile to point it out. I have no good ideas as to how the developers could improve Blind in PvP without changing how accuracy functions as a whole. This is obviously not something that can be done in a patch, but the sole thought that comes to mind for improving the effectiveness of Blind is just that, changing accuracy. This is just a thought, but in PvP, accuracy perhaps should serve a completely different role than what it does in PvE, perhaps in the form of piercing defenses, or a means to enhance PvP abilities, or even a secondary form of crit, but leaving core abilities to four Jobs as useless in the place where they should have the most use doesn't seem right. And while yes, I only mentioned Dark Passenger and Flash, let us not forget about Ruin II.
Next, is probably the ability that gets the second most grief for Dark Knight's core kit is Delirium. For those who have browsed the tank forums in the past, you likely know that I find Delirium horribly weak for what it is. As a replacement for Dragon Kick it spectacularly fails at being that, since it not only is a DPS loss in ideal conditions, but also is flat out replaced by Dragon Kick. However, there's more to it than that in PvP, I've noticed that in the Feast, the difference in my effectiveness varies wildly based on if I am facing a Bard or Machinist vs a Summoner or Black Mage, for one reason: Delirium. Dark Knights from what I have witnessed are incredible anti-caster tanks, where Paladin easily crushes enemy melee, Dark Knight does the same to casters, but not to rangers. A simple fix would be to just make Delirium apply -DEX in PvP only, allowing the Dark Knight to do their role regardless of the type of ranged DPS they're against, but some may consider that too good. Beyond this, and I've suggested it in the past, perhaps Dark Knight should be adjusted to be a pure magic damage tank and have Delirium be their version of Storm's Eye/Dancing Edge. But that's a completely different topic. In either case, Delirium is weak, it shouldn't only be better than Power Slash because it has 10 more potency as a full combo. In fact, without an enemy caster, Power Slash is arguably better since its burst is higher at 300 vs 280.
The final, and most painful weakness, one that is still tormenting Dark Knight to this day, is Living Dead. Living Dead in PvP is almost always just delaying the inevitable. Holmgang has amazing offensive use and pinch defensive use, Hallowed Ground, while only available once a match, is unbelievably strong, and Living Dead is just wasting time. I've won my fair share of matches by using Living Dead in the last 10-20 seconds, but that shouldn't be the only place where it feels good to use my ultimate ability. Now that tanks have even more HP than ever, I find that even in PvE, my healers simply can not keep me alive without Benediction or a lot of cooldown usage from us both. Perhaps its time to figure out a healthier way of removing the debuff, at least in PvP where communication is more strained for solo queue and cooldowns are spent often long before some other abilities. Living Dead needs something, anything to make it stronger and worth using beyond securing a victory late in a match.