Chaplains were originally formed by [[Lorgar]] as part of the religious slant he brought to the [[Word Bearers]], charging them with spreading the cult of the Lectitio Divinatus, his early form of [[God-Emperor of Mankind|Emperor]]-worship. After assuming control of the Council of [[Terra]], [[Malcador the Sigillite]] instructed the Space Marine Legions to employ Chaplains to enforce the Decree of Nikaea, the Emperor's restrictions on [[Librarians]], as a sort of proto-[[Commissar]].
[[File:DreadnoughtChaplain01.jpg|300px|thumb|left|This Chaplain is a dreadnaught, [[Chaos Space Marines|your]] [[Heresy|argument]] is invalid]]
In the present, a Chaplain's role in a chapter is to root out [[heresy]], inspire other Marines by chanting a litany of verses, maintain the Chapter's relics and traditions, and basically make the Marines around him righteously angry. Unlike [[Commissar]]s of the Imperial Guard, he isn't keen on executing his own men for simple cowardice, mainly because Space Marines only retreat to gain a better tactical advantage and the fact that a Marine is more expensive to create than an expendable guardsman (plus the whole "And they shall know no fear"); but he does still torture/kill [[xenos]], mutants and heretics for tactical, religious, or recreational purposes. Chaplains are an HQ unit and use a Crozius Arcanum. It is their badge of office, a fuckhueg [[power weapon|energy-wreathed mace]] capable of going straight through armour and utterly shattering your head right off of your shoulders in a single swipe. All the while looking totally sweet in his black Power Armour and skull visage helmet while scaring the living shit out of the enemy (Well, in fluff. On the tabletop it's just a Power Mace in 8th ed.), higher ranked Chaplains may also be issued Terminator armour. Outside of the company Chaplains, you have the Master of Sanctity or High Chaplain oversees the [[Space Marine Chapter|Chapter]] Chaplaincy and is a senior advisor to the [[Chapter Master]], and the Reclusiarch, who oversees the Reclusiam, where the Chapter's sacred relics and trophies are displayed. Many Chapters combine the two posts, but not always.
For all intents and purposes, just imagine a Space Marine, but even more fanatical and badass. He also has [[Indrick Boreale|great oratorical skills]] to make his brothers stop bitching and to pick up his knife then charge that [[Bloodthirster]] and stab him right in the balls because goddammit that's how you make the [[God-Emperor of Mankind|Emprah]] proud. [[Heresy|It is also suggested that Chaplains use their "oratory" skills for other purposes as well]], but these may just be rumours spread to question the MANLINESS of the Spess Mehreens. As an interesting aside chaplains seem to be more in-touch with their emotions than other Space Marines, seriously though if you listen to their lines in Dawn of War they sound like they're about to burst into manly tears of pride or rage; they must watch [[Fist of the North Star]] as part of training.
The [[Chaos Space Marine]] equivalent is the [[Dark Apostle]], most associated with the [[Word Bearers]]. Older fluff stated that only Word Bearers could have them because no other Chaplains ever fell or will fall to Chaos... which later editions revealed to be a lie, since Chaplains in their present form only originally existed ''in'' the Word Bearers in the first place. (They did start to appear in other Legions after the Edict of Nikaea though. And that's not counting more recent Chapters going renegade.) The [[Death Guard]] also have a sort of variants in the form of the [[Tallymen]].