Paladin

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A paladin is an old word describing a guard or a knight, it later became a word describing legendary Knights of great feats. In modern context, it generally refers to a class found in various roleplaying systems, most notably D&D. 
==Fantasy Setting==
In the fantasy setting, Paladins are, if not necessarily holy warriors, at least warriors imbued with magical powers. 
Normally Paladin adheres mainly to good characters, but in this setting, a paladin can be of any alignment and sometimes, their alignment might not even matter. 
Paladins in this setting are normally involuntary champions of a god who at some point in their life recieve something called "the calling", which is pretty much what it sounds like, an irrational desire that calls upon this individual to take upon himself some sort of duty. Some paladins almost appear mad and crazed as a result from these effects as the calling can either be a strong and compulsive urge they might initially resist, or even take the form of voices in their heads. 
Paladins gain supernatural powers from adhering to the calling, and as long as they keep doing so, these powers remain. Some paladins don't even have to actively pursue their calling, they just have to avoid opposing it, though doing such may be harder than it sounds. 
While a few paladins are definately borderline sociopathic and quite insane, the paladins best remembered are the ones who have followed a particularily rightous calling and performed noteworthy deeds.
It's hard to say anything decisively about paladins in this setting because of their diverse natures, but especially good paladins go down in history as champions of virtue and justice, often revered for their willingness to lay down their lives for a cause they uphold with a burning passion. 
Very disturbingly, the evil alligned paladins tend to burn with exactly the same passion, and while no gods are outright malevolent, not all paladins serve gods, some serve the seducing and strange alien voices of Chaos, and such paladins can be wildcards at best. 
The Empire of Daeghonia is the first force in the world to recognize that paladins exist in many shapes and forms. Many civilizations have come to understand that there are paladins serving certain gods, but do not recgonize other paladins as being the same. In Daeghonia, all paladins are officially referred to as such, but they have an order known as "the Holy order of Paladins" which is composed only of good aligned paladins. 
To avoid confusion, Paladins are often cut down the middle and divded into two groups, Paladins and Punishers, with paladins being lawful neutral, lawful good, neutral good and chaotic good and punishers being all the other alignments. The punisher title doesn't fit well with all people in this group, but most paladins of evil aligned gods or causes usually serve a function which this title describes well enough rather than being a protector of the people. 
Paladins said to be nearly unstoppable and impossible to kill and able to kill anything, attacking with incredible speed and power. Paladins have an innate ability to heal themselves and anyone around them, to cure people from illness and other afflictions. A few rare paladins can also cast a few divine spells, but this is not normal, not is it entirely considered a benefit as it seems there is a limit to the amount of supernatural power a paladin can draw upon, and being able to cast spells seems to consume quite a bit of it, leaving their other abilities weakened, and it also demands more of their general focus as spell casting is complicated. 
Punishers are essentially the opposite with the exception of true Neutral paladins. Punishers, while still able to heal themselves, can not heal others, nor cure inflictions, but rather they can hurt people at will, spreading disease in them, but they are also mighty warriors, though much less capable of protecting others. 
True Neutral paladins are by some considered the strongest, but this is only by a margin, as true neutral paladins may both hurt and heal people as they see fit, spread disease or cure 
Paladins gain a "companion", which doesn't need to be an animal companion, it can be a spirit under their command or a fiend of some sort, though the average paladin seems to favor a horse, as most of the organized paladins exist inside the Order of Paladins which is an order of Knighthood, meaning they revere mounted combat. Whether they actually summon forth a spirit or celestial or even a fiend, or relies on a living animal matters not, as no matter what, a supernatural entity will be called, but it may work through an animal if the paladin so desires. 
Paladins do not need to bind a special weapon in their service (as by pathfinder rules), but can imbue any weapon of their choice with the magical properties they seek to make use of, even their own body. 
Paladins can call upon magical energies in combat against certain foes. This is known as "smiting". When a paladin smites, the effect is subject to certain criteria, for instance, but the rules are not as strict as they normally are. Some gods in this setting, and some causes also, can have people of multiple alignments, some times even opposing alignments serving under them. A paladin who smites can do so if the enemy is at least partially in opposition to the paladins alignment, meaning a lawful good paladin can smite a chaotic good character if this is necessary to adhere to his calling. A true neutral paladin can essentially smite anyone, but might very rarely have cause to do so as very few people conflict with a true neutral paladins believes. If a true neutral paladin isn't bound directly to the alignment of neutrality itself however, they often serve a specific cause just like other non-religious paladins, and as such can only smite those who oppose their cause and their calling. 
Paladins who are holy warriors are usually much less restricted, in that anyone who basically stands in their way can be smitten, as long as they serve their god, with the exception that, anyone of the same faith as them cannot be smitten. 
If one imagines a cosmic chess board, priests and preachers are merely pawns, while a high priest might be a king or a queen, but the paladins are the Knights, bishops and towers. Gods, unlike paladins, always see the bigger picture and their plans are often so complicated that mere mortals cannot hope to understand them, as such, it's not correct but almost more fitting to say that the gods simply do not care if a few mortals live or die, and they certainly do not care if their paladins are their murderers, as long the end goal is achieved. 
Naturally evil paladins can smite whoever they feel like, maybe with the exception of people who are even more important to the cause they serve, than themselves. 
===Game terms===
What does this mean in game terms?
When you make your character, you decide on what kind of paladin you want to play and make a note of it somewhere. The GM might not be okay with just any kind of paladin in his group, because of their tenaciousness. Paladins are fully allowed to multiclass, and further more, they can adapt their class quite a bit. For instance, by giving up heavy armor and shields, as well as his will save, a paladin may optionally switch his fortitude for a reflex save, and upgrade his skills, but his class skills are dependant on what cause he serves. A paladin of the patron of thieves is likely to have disable device and slight of hands, but a paladin of the god of justice isn't. 
At some point there might be a decisive document formulated that dicates the what and hows of paladins, but so far, everyone will just have to deal with it. 
===Roleplaying hints===
If you play a paladin, get the GM to help you formulate a very strict code of conduct. Remember that your character will lose all his paladin abilities for breaking this code. Unlike in a normal setting, your character doesn't need the atonement spell to regain them, he just needs to find his way back to the path. A paladin doesn't stop feeling the call just because he loses his abilities, it just becomes all that much harder to follow his calling if he does.