The additional action classes for dark knight are gladiator and marauder. There is a switching action into tank mode called Grit Stance. During this stance, damage taken and damage dealt are reduced, while enmity generation and accuracy are increased. This would be a skill that you would always want active while main tanking.
From there, there's an opening weaponskill called Hard Slash, an enmity-generating weapon skill called Spin Slash and an enmity-generating weaponskill called Power Slash which draw hate as a three stage combo. There are also buffs and debuffs which can be applied such as a DoT called Scourge and a buff called Shadow Wall which reduces damage taken.
Low Blow is a single target technique to jump and attack the enemy (with a Stun effect) with a cool and unique kick animation. Additionally, Dark Knight has a variety of abilities including single target (Unmade) and AoE magic (Unleashed) that can increase enmity.
Salt the Earth is a ground targeted AoE skill positioned with the cursor which has a non-elemental magic attack that hits targeted enemies.
Additionally, there are multiple combo paths which have different special effects depending on the route. Some of these have MP recovery at the second stage so they can also be effective as two-step combos such as Hard Slash to Siphon Strike. After Siphon Strike, it's possible to continue with Soul Eater (HP absorption based on damage taken, with the amount depending on certain conditions) or Delirium Blade. The combo has a debuffing effect which weakens the enemy somehow - the effect was still undergoing last minute adjustments.
The basis of dark knight which we'll describe in more detail below, and the element which will require the most skill, is utilizing the Darkness technique which consumes MP and balancing it with MP recovery techniques.
Blood Price is a MP recovery technique which allows the dark knight to recover MP when attacked. There's also an action which allows the dark knight to recover MP from his own attacks.
The Darkness action that makes a dark knight a dark knight is learned at level 30. However, this state consumes MP for increased damage and things like Mage's Ballad will have no effect (Yoshida said "you cannot be aided by others"). It's also possible to stack this with Grit Stance and since your natural MP recovery is higher than the MP consumption when you're out of battle, you should pretty much have it on all the time. Although you become stronger in this state, you'll become stronger still when you utilize Dark Arts which consumes large amounts of MP.
There are powerful new abilities that can only be used while using Dark Arts, and these will be used often. However, since MP consumption is considerably increased with Dark Arts, the margin for optimal play will be much tighter. Dark Arts does not raise attack power itself, but raises the power of individual actions or adds special additional effects when the actions are used. For example, while using Dark Arts, the weaponskills Soul Eater and Abyssal Drain are enhanced with the additional effect of absorbing HP. The defensive buff Dark Mind receives an increased amount of damage reduction.
The new Living Dead corresponds to Holmgang from Warrior, but the name suggests a slightly different effect which will set it apart. Please check this out for yourself.
For main tanking, the flow of battle will be to activate Darkness, set Grit Stance, grab the enemy with a single target magic attack, and then close the gap with Plunge Cut. From there, while earning hostility you can perform various combos interwoven with the Dark Arts state.
Actually, that series of actions should get you through the new instanced dungeons and high rank monster hunts without any problems once you understand the mechanics.