We have talked about how to unlock the Dark Knight in the article before. For more information about the Dark Knight class in FFXIV, I'd like to share with you as below:

Dark Knight, first officially announced during the 2014 London Fan Festival, is a Tank job that wields a Greatsword (2 - handed sword) as its weapon. In battle, Dark Knights use magicks to infuse their blades with the powers of darkness to strike down their enemies and defend their allies. Like with other jobs introduced in the expansion, Dark Knight will have no base class and will start at level 30.

Abilities:

Darkside

Similar to a paladin's Sword Oath, this stance is more of an attack - based stance.

Players take on an aura - like effect known as Darkside, while this is in effect MP will deplete gradually. While in this stance, players will not be able to receive MP healing support from party members.

Grit

This stance will be used when in the tank role. With this stance, it'll be easier to gain enmity.

Living Dead

Similar to Holmgang, this is an all attack blocking action. It applies a 20 second buff that, if hit fatally, allows one to survive and gives ten seconds (of a "zombie" state) to be healed to full HP. If time runs out the player will be KO'd. Players cannot die during the Pre - buff or the Zombie state. Zombie is cleansed as soon as HP is at full.

FFXIV Heavensward: Dark Knight Ability Overview

FFXIV: Heavensward - Dark Knight All Job Skills and Abilities Guide