DRK was the first thing I picked up in Heavensward. The class is basically a reskinned PLD (look cooldown structure) with added flavor of WAR (look combo rotation usage). The class isn't as complex or overwhelming as people make it. It is also nicely paced for people to pick up the mechanics as they go learning the class.
At 30, you get Grit and Darkside giving you your actual tank stance and the damage buff for the MP drain vs upkeep niche.
Until 45, you basically are a PLD with a damaging flash and a semi-life-stealing Soul Eater.
After 45, you get Dark Arts and you can finally spend your MP for something useful, this is when your options start to branch, but you will mostly spend Dark Arts on either Dark Passenger in AoE or Soul Eater on single target. Dark Mind and Dark Dance are of no noticeable value yet. Blood Price becomes really valuable but all you have to do is pop it off cooldown.
At 50 you gain living dead and Delirium and your basic rotation is completed by having all 3 basic combos.
At 54 you get Plunge, making you the only tank with a charge attack and, if timed well, the most frequently used anti-knock-back tool.
At 56 you get Abyssal Drain, the better AoE version of Unleash and can be couple with Dark Arts for self-healing.
At 60 you get another Dark Arts spender, Carve and Spit. Your decisions change slightly by dropping DA from 1 Soul Eater in favor of pumping up C&S and you use Delirium instead. I personally like to use C&S for MP in AOE pulls because it literally gives me an additional Abyssal Drain. Your decision making hasn't changed much except should I pump 350 more potency of get MP back.
At 60, Dark Mind becomes relevant since magic damage tank busters are heavy one you start doing Bismark Ex and Alexander. But if by this point you have trouble doing anything outside of Siphon Strike combos then you have bigger problems.
Dark Dance also becomes relevant since it helps with speed running. DA DP + DA DD and you have a massive dodge chance in an AoE pull. Dark Dance sucks for mitigating physical tank busters and should not be counted on for such cases. Popping it to force a parry for reprisal and the chance of parrying the physical buster is a smart idea, nonetheless.
EDIT: For all that's holy, do NOT Dark Arts Dark Dance on bosses, you cannot dodge boss attacks.
Your basic CD rotation is the same as PLD but instead of completely relying on the big CDs you couple in small ones (Dark Dance or Dark Mind) to enhance the effect.
EDIT: If you look at the class at its basic level, it is simple. The class is as simple as PLD if you just want to function at the basic level (hold aggro and pop CD to mitigate damage). The class's "complexity" becomes apparent if you start trying to maximize the damage you deal while performing the basic function of holding aggro and reducing incoming damage.
I did not add skills that do not change the mechanical core play or the decision making. Salted Earth should be used off CD for added damage. I also skipped 58 Soul Survivor for the same reason.
Blood Price is useful in every situation you're taking damage. You do not use as much MP on single target as you do in AoE so the MP you get in ST, while far less than in big pulls, is still relevant to the MP you spend.