The Dark Knight is a tank class which was introduced in Heavensward 3.0, and have a special requirements (Like Astrologian and Machinist) compared to the original jobs. If you’re a newer DRK, the Dark Knight leveling guide may be useful.

How do I become a Dark Knight in FFXIV? What are the DRK Requirements?
1) You must have an existing Level 50 combat class (DoW or DoM).
2) You must have completed “Before the Dawn” (all story quests prior to 3.0).
3) Only then will the Dark Knight unlock quest open up

Where do I unlock Dark Knight?
The quest name is “Our End”, which is found in The Pillars (13,8). The questgiver is an NPC names ‘Ishgardian Citizen’.Help! I’m a new player rushing to Dark Knight! Then you’re best off starting the game as Gladiator and hitting the requirements as a Paladin. Then switch to DRK once you’re good to go.

Dark Knight stats: What stats to allocate and gear up for?
DRK’s, being a tank class, have only two stats to choose from. VIT will be the go-to choice for 80% of Dark Knights, and would be correct most of the time. Many tank players like to allocate STR to increase their DPS. This may not be the optimal choice depending on your level of play, and your static group. But either way VIT, STR or a mix of both would be the way to go.

What are the Dark Knight Skills & Traits?
Unlike other jobs, you jump straight into DRK at level 30. So you might get a bit overwhelmed. The skills and traits are listed near the bottom of the post.

What classes are Dark Knight Cross-class skills from?
Gladiator and Marauder. (More info: DRK Cross-class skills)

Crafting & Gathering classes suitable for Dark Knight?
Going by trends we can assume…
DRK Armor: Armorsmith
DRK Greatsword: Blacksmith
DRK Accessories: Goldsmith

For gathering, Mining supports all the above crafters quite well, which is quite fortunate for DRK and other heavy armor users.