Dark Master is a Endgame Boss.
Dark Master (World Tier IV): Defeat a World Boss on World Tier IV or complete a Legion Event. From these activities you’ll acquire Exquisite Blood. Once you have enough, you’ll be able to summon the Dark Master. The Diadem of the Ancient Helm Cosmetic can potentially drop from this boss, in addition to Unique items exclusive to it.
Keep in mind that: You’ll need to meet the summoning conditions of a boss each time you want to fight it, so hold on to any additional summoning materials that you might acquire during your travels.