The Dark Knights (otherwise known as Nazgulz) are available as a raid in Skyrock, and require the Sacred Souls scroll to be in your inventory as a pre-requisite. The raid can be found on the bottom floor of Skyrock in the upper-right, and requires 3 total players.
To increase your chances in the raid, ensure that everyone uses their scrolls to keep up the attack power buff (400%) for 90s, and use healing scrolls and mana potions to keep up the damaging skills. The enemy attack patterns are fairly simple to deal with, but with 3 enemies on screen they can be hard to deal with - leave the Warrior for last, as his attacks are the simplest to predict and avoid. Archers can deal with this raid the best, as most attacks are not ranged (except for the Mage).