The basics to know for DRK
Dark knight is one of the most damaging jobs in the game. They use their heavy melee along with some nice black magic. As a DRK you must be aware that your heavy damage comes at a small price. DRK's don't have a whole lot of defense; decent but not great. You will draw a good bit of hate with your damage at times, so don't dish out more than your tank can handle. You are made for melee primary and mage secondary; don't rely on your magic more than your melee. And finally you are not made for tanking although at lower levels you may be asked to if nothing else is available.

WAR and THF are the most common subs for DRK, although I've seen RDM, BLM, MNK, and DRG subbed. Experiment. Go with what feels right to you. Subbing WAR or THF will more than likely get you an invite quicker.

Dark Knight Subjobs
If you want to jump on the bandwagon, this is how you will manage your subjob:
Levels 1-29: Usually WAR or THF. Either is fine.
Levels 30-49: WAR or THF. WAR get Berserk and THF get Sneak Attack.
Your call.
Levels 50-59: WAR for Berserk and Double Attack
Levels 60+: THF for Sneak Attack and Trick Attack OR WAR still
works great usually until level 66

Dark Knight Races and Subjobs
I'll start out this section by saying there is no wrong race to pick for being a Dark Knight. So don't worry if you're a Tarutaru wanting to be a DRK. Any well-equipped race can keep up in damage to another race.

Can I be a Taru DRK?
Of course you can. I've seen well equipped Taru DRK do an excellent job and the damage is surprising. A RDM or BLM subjob in addition to WAR and THF are great for Taru DRKs.

Mithra DRK?
Mithra DRK will have a slightly higher hitrate than the other races. Common subs include THF, NIN, and WAR. Although a mage sub wouldn't be incredibly bad.

Hume DRK?
Most people will choose a Hume because they're well-balanced in every stat. Again, common subs are WAR and THF. Mage subjob is still in the running, but not too common.

Elvaan DRK
Elvaan DRK have the most STR, but miss the most of all races. You can counter that though by being well-equipped. Common subs are WAR or THF. Mage sub is probably not a good idea here. Just get some pieces of RSE and that should do you good for MP.

Galka DRK
Galka are great DRK, but lack in the MP department. Best way again to counter this: RSE. Mage sub is not a good idea. Common subs: WAR or THF

These are just ideas to give you a hint of what to expect. I'm not telling you this is the way it HAS to be done. SAM can also be a good sub, but I didn't list it here because it's not common.

Soloing your DRK
DRK have a pretty nice edge when soloing. You do great damage to make up for the delay of your weapon. Usually the mob gets in 2 hits for your 1 hit. You can take on even matches until around level 15 or so, then it gets tough. You have many ways of regaining the edge in a battle if it looks grim. Blood Weapon + Souleater is a great combo when soloing if you get in trouble. Souleater takes the HP away while Blood Weapon gives it right back to you. You also get Bind and Sleep, which bodes useful for a quick getaway.

DRK Two Hour: Blood Weapon
Pretty much only useful for soloing. You could help out the party in certain emergency situations by using Souleater and Blood Weapon simultaneously.

DRK's in Parties
DO NOT do more damage than your tank can handle. Too much damage makes it hard for your tank to keep hate. A Paladin is a Dark Knight's best friend. This duo can benefit a party greatly. A good PLD can keep hate from the DRK's massive damage. If you don't have a PLD or a tank that can keep hate very well, you'll find it hard to use your abilities without accruing hate. You definitely need to keep in mind that you should NEVER stack SOULEATER, BERSERK, AND LAST RESORT. This will cause massive hate towards you and you will more than likely die. The only exception is in an emergency.

Example: Your BLM Magic Bursts for 500 damage. The mob goes after the BLM. Turn on Souleater, Berserk, and Last Resort and hit the mob a few times. He'll turn to you and the BLM will be safe, long enough for the tank to get hate back.

In the case of a NIN tank, you will find it a completely different playing style than with a PLD tank. You have to calm yourself down a little. You can't unleash as you normally would with a PLD tank. I'm not saying NIN tanks are bad. As a matter of fact, I think they're awesome. The mages use little to no MP. Just be careful when having a NIN tank in your party and don't overdo it too much, unless he/she can keep hate.

Dark Knights get parties pretty easily. They are also good to a party when fighting BCNM's 40 because of their Stun Spell at lvl 37.

Spells Worth Getting

Bio - Useful at lower levels. Great for raising Dark Magic Skill.
Drain - Incredibly useful. Saves the healer's MP, heals you, and damages the mob all at the same time. Usually equivalent to a melee hit, depending on the mob's resistance and your Dark Magic Skill and INT.
Aspir - Incredibly useful. Gets your MP back so you don't have to rest in between battles. Slows downtime and conserves your TP.
Poison - Very useful. Excellent for pulling. Doesn't do a lot of damage at all, but it's quick cast time. Superb pulling spell and excellent to level up your Enfeebling skills.
Sleep - Only useful if you have high Enfeebling skills. In the case you have an emergency and need to zone, Sleep will help out tremendously (granted your Enfeebling skills are up to par). Also useful for certain BCNM.
Bind - Very useful. Pretty much same reason as Sleep. Very cheap too, so just go ahead and buy it.
Stun - Incredibly useful. Especially since DRK's get it before BLM's. It can prevent mobs from doing naughty things. Also highly useful for 40 BCNM.
Absorb spells - These are the DRK signature spells. They all help your party and are excellent to have. The only one I haven't found useful is Absorb-CHR (if anyone brings about a good use for this, let me know and I'll add it). They all are fairly cheap, except Absorb-DEX. This runs about 250k-300k gil. You should plan on getting this spell though, especially if you're Elvaan. It will help you a good deal.

Absorb-CHR - Haven't found a use for it yet, unless you were fighting Bard or Beastmaster mobs.
Absorb-MND - Helps your mages. They can stick enfeebles on the mob much easier without getting resisted as much. Also lowers its magic defense.
Absorb-INT - Good for if you're fighting BLM mobs. Lowers its damage done by magic.
Absorb-AGI - Very useful and helps your melee. Lowers its AGI while making the hit percentage higher.
Absorb-DEX - Useful if you seem to miss a lot (which most do unless you're Mithra). Also increases your chance of a critical hit.
Absorb-STR - Very nice. If you don't sub WAR (and even if you do), this is like having Berserk or another Last Resort.
Absorb-VIT - Helps out your melee. Lowers it's Defense and adds to yours. Melee does more damage.

Elemental spells - I left these out of the important spells because I haven't really found them that useful. They don't do as much damage as a melee hit, so why waste time casting these when you could be doing more damage? Of course you can MB, but I haven't found it does much damage either. Maybe when I get up to the spells like Water II and Fire II, I'll like them more. Thanks Danalog the Vengeful Programmer

DRK Equipment
Scythe vs. Great Sword: Scythe or Great Sword is ideal for a Dark Knight. While a DRK has an A+ rating using Scythes and an A- rating using Great Swords, you won't notice any difference until around level 60+. I would say definitely keep both skills capped. You can do better skillchains depending on who you party with. Scythes are usually cheaper than Great Swords at lower levels, but if you talk to Gumbah again after killing the first 100 mobs, then kill 100 more mobs, you will get a nice Great Sword that has lvl 5 requirement with 20 DMG and +8 to attack only useable by DRK's. ALWAYS update your weapon before you update your armor. You were made to deal damage not look pretty.

Armor: Keep your armor fairly updated. You can get by every now and then by skipping an armor update. Usually you can sacrifice armor with higher defense for armor that benefits your stats. I personally am a perfectionist and HAVE to have current armor, but that's just me. Thanks trickybeck

DRK Stats
Dark Knights only need to focus on 3 main stats: STR, DEX, and INT. You don't need to worry about INT that much though because you won't be relying on your magic as much as your melee. As far as STR and DEX go, it depends on your race. If your race is naturally low in the STR field, invest in +STR and +Attack items. Meat Mithkabobs are excellent to use (even though they are food and not equipment). If your race is naturally low in DEX, invest in some +DEX and +Accuracy items. If your race is pretty much balanced in STR and DEX, invest in both. Stats like AGI, CHR, and MND are not important.

NOTE: The bolded statement above is mainly pre-33.
Once you get post-33, it'd be wise to start investing in +Accuracy and +STR items.

1 Attack = 2 STR
1 Accuracy = 2 DEX
Accuracy > Attack

Unlocking the Job
Go to Bastok Mines and talk to Gumbah. He's in one of the houses on the second floor. Talk to him until you get the cutscene. He tells you to go to Palborough Mines and talk to a Galka named Zeid. Go to Palborough Mines and take the elevator up to level 3 of the mines. Get on the boat to get the cutscene with Zeid. Zeid then gives you a very crappy sword (3 dmg, 666 delay) and tells you to satisfy the sword's hunger or something. All you have to do now is kill 100 enemies with that sword. Go to North or South Gustaberg and just kill 100 of the easy mobs outthere. You have to keep count because the game doesn't keep count for you. When you've killed 100 mobs, go to Beadeaux in the southeastcorner of Pashhow Marshlands to activate another cutscene with Zeid. Congratulations, you can now become a Dark Knight! Thanks
CloakedStranger

NOTE: You can talk to Gumbah again and kill 100 more mobs with the same sword, then trade it to the pond in Gusgen Mines to get a great sword called Deathbringer.

Closing
Being a Dark Knight is an exciting experience. Who doesn't like being able to deal that much damage? Just be careful of TOO MUCH damage. Have fun with it!

Note: If you have something I missed feel free to let me know. Thanks for taking the time to read my guide.