Blackguard

Fuck being misunderstood, he wants you to see how much he enjoys kicking puppies and crapping on your lawn.

While Paladins encompass everything Noblebright and good-hearted, Blackguards, often called antipaladins by players, serve as the spiritual opposite to their pure-spirited and benevolent counterparts. Blackguards are known to affiliate themselves with demons and serve dark, generally malevolent deities, and are described as being hated by all other races and classes that serve good. Blackguards are allowed to perform a variety of malignant actions such as using minions to do his scrimpy work for them, using sneaky and backstabbing tactics to get the better of their allies and/or enemies, or generally bully those who serve the forces of good in order to assert his overwhelmingly evil and mean-spirited dominance over others.

Non-playable Blackguards are usually found either in positions of leadership, such as leading hordes of Undead and other nasties within their posses in order to throw eggs and post dog shit through all the doors across their targeted realm, or serving as kiss-asses dark lieutenants for even more unpleasant and villainous entities and characters, which they know full well could end their mean-streaks by sending them to their rooms in a plane of torment and imprisonment and throwing away the key. Blackguards can also be seen as lone wanderers, either working as sell-swords due to having taken the "money is the root of all evil" philosophy in literal context or just skulking around committing crimes and making public spectacles for the sheer hell of it. To become a Blackguard, the character in question must have made a pact, or at least peaceful contact, with a summoned evil, usually an Eldritch abomination of some kind.

Blackguards in D&D

Blackguards were introduced to the Dungeons and Dragons setting through the 3.5 Edition of the game, published in 2003. Blackguards were an available class for fuckup Paladins who had grown tired of doing good and instead wanting to unleash their inner rage against all those DMs who took advantage of their good-natured alignment, being accessed via multiclassing. Becoming a Blackguard grants the player several bonuses if the character decides to trade Paladin levels.

Blackguards are automatically proficient with with most weapons and all available shields and armors, and they emanate an evil aura that grows in power according to a Blackguard's level. Blackguards are so evil that they are capable of outright detecting those who fall under the alignment of good, primarily because wherever they go good deeds happen to be an extreme rarity, and they are incapable of accidentally poisoning themselves when applying toxins and venoms to blades. Blackguards are allowed to smite good once a day through the use of a melee attack, usually wasting this opportunity by either punching anything cute and cuddly or lashing out at their snobby, do-gooder opposites, and their aura of evil deals a -2 penalty against saving throws against all enemies within a vicinity of 10 feet.

Blackguards in Pathfinder

Blackguards are a "variant class" in the Pathfinder setting, which basically take the Paladin and reverse it to create a demon-worshipping, Chaotic Evil monster.

Oathbreaker Paladin

In Dungeons & Dragons 5th Edition, character alignment got a lot less crunch assigned to it. So Paladins are no longer as vulnerable to getting their powers yanked as they were. However, they still uphold various sacred oaths to get those powers, and so the Blackguard lives on as the "Oathbreaker Paladin" class variant. He gets a bunch of "evil-themed" warlock and necromancy spells, like animate dead and hellish rebuke, control undead and dreadful aspect forms for Channel Divinity, an aura that supes up friendly undead and fiends, resistance to non-magical physical damage, and a suped-up aura of heartstopping terror.

An Oathbreaker can attempt to redeem themselves, but once they succeed, they're on their last chance; a 5e Paladin who breaks their Oath and falls a second time after becoming an Oathbreaker Paladin once can never become any sort of Paladin again... other than an Oathbreaker.