Oathbreaker

An Oathbreaker paladin was one who had forsaken the divine oath they made, for a path in life of treachery, selfishness, or some other evil ends.

"Darkness will obey your summons. The dead will march to your rhythm. Even now, these shadows gather around you. They sense cracks in your armor. They yearn to be used, to be inflicted."

- The Oathbreaker Knight.

Description
It was said that the heart of a paladin who broke their oath held no light within, but was filled with utter darkness.

Oathbreaker paladins who truly and wholly atoned for their misdeeds could return to the light. They shed off the powers of darkness and reclaimed their previous, or even an entirely new, oath as their own. A second broken oath would return them to the path of the oathbreaker, and was considered an entirely unredeemable act.

Abilities
Unlike other paladins that were capable of rebuking them, oathbreakers gained the ability to control undead beings. One or more times per day, an oathbreaker paladin could attempt to gain complete control over a single undead creature for a single day's time.

Oathbreaker paladins gained an aura of terrible dread that surrounded them. They could channel that dread into a burst of supernatural terror that afflicted any creature within around them.

As oathbreakers continued to hone and harness the malice within their hearts, their combat prowess, along with those of malign allies, were made more potent and devastating. Any fiends or undead beings within a certain distance of the paladin fought with increased ferociousness. The extent of this power grew over time, as the oathbreaker garnered more power and skill. Once the paladin themself reached a certain level of expertise, they could shrug off most damage inflicted upon them by non-magical sources.

The most powerful oathbreaker paladins became the epitome of darkness and dread. They could surround themselves with auras of shadows that siphoned away light around them, and afflict pain the psches of anyone within caught in the throes of horror. The oathbreaker could even last out at their enemies with those shadows, which were imbued with harmful necrotic energy.

History
Throughout the ages, paladins who forsook their oaths were visited by the Oathbreaker Knight, a once-vassal to a corrupt lord who was the first individual to follow the path of the oathbreaker. The knight offered to those individuals the powers of fear and darkness he claimed for himself so long ago.

Notable Oathbreaker Paladins

 * Oathbreaker Knight, the first paladin of the Realms to ever break their oath. The knight offered the powers of the oathbreaker to paladins that veered away from the path of righteousness and virtue.
 * Arkhan the Cruel, a dragonborn oathbreaker paladin of Tiamat. After a brief visit to the world of Exandria, Arkhan replaced his left hand with a legendary artifact known as the Hand of Vecna, with the intention of using its power to free Tiamat from her prison in Avernus.