Death Knight (class)
Death Knights are typically former Knights who have turned to the Dark Side - or (former) apprentices of other Death Knights. Completely self-made Death Knights with no past as actual Knights or previous contact with other Death Knights is very rare.
Death Knights - as their name suggests - are the dark equivalents of Knights: spellcasting warriors who have their own rules and laws.
They use Arcane Magic.
Death Knights are almost as old as knights themselves - almost, coming into existence merely half a century after them. The first dark knight made his appearence in the early 2nd century AEKE, corrupted by a necromancer - said Death Knight also became the first publicly known vampire, while his mentor the first publicly known lich. These two - with their undead allies - conducted the famous Knights' Massacre in 102.
What followed the event was the Expulsion of the Undead in 104, which led to the creation of the Republic of Keldorn. This, however, did not lead to the creation of any order and structure among Death Knights - as a matter of fact, lone and stray Death Knights were often considered a threat to the regime in Keldorn, being corrupted ex-Knights, or former apprentices of other Death Knights, wandering around and looking for an opportunity to either rob the weaker or get hired for money to do someone else's dirty work. Death Knights also remained an ever-present menace in Etrand as well - the survivors of the massacre of 102 were unwilling to start a new order, so they largerly had individual masters wandering the world, training individual apprentices into proto-Knights - this lack of standardization and regulation led to many apprentices, even masters being suspectible to the corruption of the Dark Side, hence corruption into Death Knights - Death Knights remained more common in Etrand than in Keldorn, with Keldornish Death Knights finding it difficult to find an apprentice, having the replenishment only via escappes from Etrand.
This all changed in 525, when Worgand the Feared founded the Order of the Knights of Death, just 9 years before an event - courtesy of Ashrám Wegnard, another famous Death Knight - that shook the reputation of the Knights of the Blood Red Light, leading to many fresh converts to the Dark Side more than willing to make the journey to Keldorn and join the order. Even before said event, there was a surge of defections from the light-oriented knightly orders during the late 4th and 5th centuries, which warranted the organization of these people into an order. After the assassination of Worgand, the order itself went into a decline (from which it since has recovered, courtesy of Burcas Hordung), but not the lifestyle and the profession - the profession of being a Death Knight travelled, not just abroad, but to other organizations, profiliating amongst the various vampire clans and werewolf packs.
Since the 8th century, Death Knights had a renaissance, with people like Héli Calbae making it known to the Followers of the Light, that the Forces of Darkness are far from defeated, and will always find a way to feed off even from the light.
Appearences in media
Death Knights are a prestige closely related to Knights, in the sense that they have the same prequsites as Knights, with the exception that they have to be any Evil alignment or Chaotic Neutral.
Being a Death Knight comes with the bonuses of receiving all armour skills, an advanced skill of archery, and regardless of their alignment, they are good at Dark Magic.
In the video game adaptations of the FRPG, Death Knights have the exact same amount of hitpoints, spellpoints, base attack and defense values as their light counterparts, but they have a vastly different selection of spells, as all healing spells are replaced by curses and other spells of dark nature. In addition to that, they can learn spells from scrolls like Magicians, and unlike Knights, they are allowed to use ranged weapons.