Hello there. This will be a guide on the class 'Dark Knight' for those who want to try and is unfamiliar to the class. I won't go too much in depth as I, myself don't have boss gears yet. This is for pre-awakening or Lv.56 below.

Gear

Armor

Get full Grunil asap. If you are grinding or getting a lot of pocket money, consider making them ultimate by doing item reform via the Black Spirit. 

Ultimate reform stones cost ~5 mil and ~7mil for armor and weapon respectively. They give a nice boost to your AP/DP count and should give you an extra edge.

Accessories

For earrings, you can use Elisha for extra Energy Points (Mana) or Bares for damage.
For belt, use Bares so you can take home more coin while getting extra AP (dmg) points.
For ring, use Bares, then upgrade to either Red or Blue Coral Ring.
For necklace, you can use either Yuria (more damage vs human mobs) or Bares (more damage overall).

You can use these accessories until you get Asula's Accessories. Alternatively, Treant Accsessories are not bad too.

Weapon

You can choose between these 3 when starting.
Rosar for that nice base damage.
Bares for fighting against monsters.
Yuria for fighting against human mobs.

If you have a big silver coin storage, go ahead and get a Liverto. However, you can't make this into ultimate grade so I wish you luck on upgrading it.

Sub - Weapon

Go ahead and choose any from the start but it is recommended to get Saiyer Knot afterwards.

Magic Crystals

No specific names but try to get those with the ff. effects

Helmet - Evasion and HP count or MP Count (alternately HP or MP Recovery)
Armor -  MP count and MP recovery
Gloves - Atk Speed and Atk Speed or AP count
Shoes - Movement Speed, LT limit ++ or Luck
Weapon - AP count or Atk Speed
Sub-Weapon - AP Count or Additional Critical Damage

Outfit (focus on this last.)

If you wonder why I even suggest giving an outfit guide, it's because they can give bonuses very useful and its best to get them asap after you complete your gear. Some of these bonuses are weapon durability consumption -10%, luck +1 (useful for drops), and of course looking good.

I highly suggest getting Sin Terna Prem set off MP (if you can) and then getting other outfits afterwards.

Skills

Get These Ones (No particular Order)

Dusk - For that evasion
Scarring Slash - spin 2 win
Unveiled Dagger - I love it and so will you
Hidden Strike - it's hidden alright!
Air Strike - feels like an attack in some other game
Smokey Haze - your first move to start combos.
Sudden Attack - use after smokey haze
Repeated Anhilation - rinse and repeat
Pervasive Darkness - use in between combos if you want your target to die faster
Enforcement - force your power against those mobs
Nocturne - your other evasive move. useful for back attack which deals more damage. USE WISELY.
Slanted Balance - make your opponents slanted.
Wheel of Fortune - spin 2 win v2.0

Sounds alot but you can play DK with these alone. You can get others later in the game.

Lyf Skills

You should try to prioritize getting Strength. This gives you additional carrying weight limits. More in depth here.

Gameplay

Buy a decent amount of Health and Mana (Energy) Potions every time you go out and grind. Medium sized is fine and probably the best potion size.

Buy some food too. It is suggested to buy those that give you +AP or +Combat or +Skill exp while you grind.

Make sure to use Dusk because it gives you some amount of MP (or Energy Point, I'm pretty inconsistent) and lets you dodge attacks.

My basic combo is Smokey Haze (shift + W) then use Unveiled Dagger (F). You can go ahead and add other attacks for your liking. Using Wheel of Fortune after is nice if you got stuck around the mob and should be useful as you spin your beautiful character while killing those mobs.

Corrupt Ground skill is nice but it is not advised to spam it for it is a very slow move. Dodge and use potions instead.

For PvP advice, I cannot give much because I don't do it myself. However if you are very well experienced in using the mobility skills, you should be able to last or escape. :D

I hope you enjoy playing Dark Knight and I wish you luck in your journey. I am open for corrections so please don't hesitate to comment.