Death Knight

Death knights (introduced in Wrath of the Lich King) boast powerful melee abilities as well as plate armor. These warriors supplement their strength with dark magic. Calling upon a rune system of magic, the death knight may summon unholy, blood, and frost spells. The criterion for creating a death knight previously required the existence of a level 55+ character on the player's account on any realm. However, as of December 2014, that requirement has been removed.

The death knight is the first hero class in World of Warcraft. Death knights start at level 55 in Acherus: The Ebon Hold over the Eastern Plaguelands, with multiple spells and abilities ready to use, and a set of uncommon gear. When departing Ebon Hold, they have a full set of rare gear ready for Outland. Through the death knight-specific quests in this necropolis and the surrounding areas, they learn to master the power of the death knight and learn to use this new power for the will of the Lich King. Quests take them outside the necropolis and into an area within the Scarlet Enclave.

Before Patch 5.3.0, players could only create one death knight per realm, so long as they had a level 55 character already and Wrath of the Lich King on their account. Now players are able to create multiple death knights on one realm. As of patch 8.3, the  Pandaren and the recently introduced  Allied races are now able to become death knights.

Lore
Death knight is a name shared by several organizations of powerful necromancers. These orders share a few things in common, including riding horses with horned skulls and many of the same abilities. Playable death knights, however, are specifically death knights of the Scourge.

Immortal Soldiers
The original death knights were created for Orgrim Doomhammer by Gul'dan as powerful soldiers of the Horde. These death knights were created by placing the souls of slain Shadow Council warlocks into the corpses of fallen Stormwind knights, the first of whom was Teron Gorefiend. Unlike modern death knights of the Scourge, these ghoulish fiends were not battle-hardened warriors; they were insidious necromancers who possessed superior intellect and tremendous magical power. They often favored the use of terror tactics and reanimated the corpses of enemy soldiers who fell in battle to serve them as mindless undead minions.

Most of these death knights were destroyed during and after the Second War, either killed by the Alliance or transformed into liches by Kil'jaeden.

Champions of the Lich King
Years after the destruction of Draenor, the immensely powerful Lich King created a new breed of death knights: malevolent, rune-wielding warriors of the Scourge. The first and greatest of these was the Lich King's chosen champion, Prince Arthas Menethil, once a mighty paladin of the Silver Hand who sacrificed his soul to claim the runeblade Frostmourne in a desperate bid to save his people. The rest are primarily made up of other fallen paladins whose souls were twisted and bound to the will of the Frozen Throne.

Unlike Gul'dan's death knights, these dark champions possess unholy strength; however, they do not possess free will and their minds are inexorably entwined with and dominated by the Lich King's vast consciousness. Despite the potential loss of free will, some powerful mortals are intrigued by the promise of immortality and pledge their souls freely into the Lich King's service to achieve it. Baron Rivendare is an example of this.

In the years since Arthas shattered the Frozen Throne and merged with the Lich King, the power and fury of the death knights has only grown. Now these unrelenting crusaders of the damned eagerly await the Lich King's command to unleash their fury on Azeroth once again. Unlike death knights of the Old Horde, the Scourge's death knights are not limited to their use of ranged spell casting abilities. In addition, these tireless death knights are considerably stronger, faster, and more agile than they were in life. However, both generations are equally destructive and terrifying to engage in the field of battle.

The Ebon Blade
A new group of death knights, the Death knights of Acherus, was later created by the Lich King to garrison the necropolis of Acherus: The Ebon Hold for the ultimate purpose of assaulting Light's Hope Chapel and destroying the Argent Dawn. In the Wrath of the Lich King expansion, these death knights are freed from the will of the Lich King and ally themselves with their former factions. Working closely under the guiding blade of Highlord Darion Mograine and the bolstered Argent Crusade, the newly-freed death knights have begun their march to Northrend.

The Knights of the Ebon Blade is a faction consisting of the renegade death knights that broke free of the Lich King's control after the battle of Light's Hope Chapel (in other words, player-created death knights). Led by Highlord Darion Mograine, the Knights of the Ebon Blade have allied themselves with the Alliance and the Horde with the help of Highlord Tirion Fordring of the Knights of the Silver Hand, and have pledged to do their part in defeating their former master, the Lich King. Their main base is Acherus: The Ebon Hold, taken from the Scourge after breaking free. It should be noted that the Knights of the Ebon Blade are not a separate player faction such as the Alliance and Horde. For the purposes of gameplay, player-created death knights still belong to either the Alliance or Horde depending on their race/faction.

Information also found in Wow wiki