Dark Apostle
The Traitor Legions have a lot of things that are basically "Evil Knockoffs!" to stuff the Loyalists get. And even the Chaplains aren't exempt. Whereas the Chaplains focus on treating the spiritual and mental health of their fellows and ensuring they stay loyal to the God-Emperor of Mankind, their Traitor Counterparts, the Dark Apostles, are twisted priests and seers of the Chaos Gods, leading the profane rituals that serve and honor their blasphemous masters.
Overview
The Dark Apostles proper are traitor Chaplains of the Word Bearers Legion. Indeed, they are the true leaders of the Word Bearers even in their current state; the warbands, or Hosts, of that Traitor Legion all operate under the authority of the Dark Council, a collection of the oldest Dark Apostles who lead the Word Bearers while Lorgar spends his time thinking about the Chaos Gods. This also makes the Word Bearers legion possibly the most dangerous of all, seeing as they are still unified (basically).
However, the name is also used for other Chaos Marine leaders, serving as a generic term for charismatic preachers and zealous orators among the servants of Chaos - in essence, a Champion gifted with leadership skills instead of pure, raw killpower, especially one who focuses on spreading the worship of Chaos (usually, but not always, Chaos Undivided) as a corrupt priest-figure.
All of these chaps carry the Accursed Crozius, the more heretical and spiky counterpart to the Chaplain's Crozius Arcanum. Due to their higher status, Dark Apostles also wear much higher quality (Read: Original and well-maintained) power armour. You ain't gonna see these daemonic Catholic Priests wearing hodge-podge Chaos Power Armour or the Mark V: Heresy Armour. Most of them we see, seem to wear the Mark IV: Maximus Armour but without the helmet and with a robe or even Warp-Forged Armour.
In 3rd edition, when Chapter Approved released the Word Bearers as part of the Codex Astartes series, it stated that Dark Apostles are the only Traitor Legion Chaplains who fell from the Emperor's light, forcing all other Traitor Legions to massacre their Chaplains. With the edition change and especially the Horus Heresy novels, that fluff has been retconned; in the days preceding the Heresy, the Emperor championed the Imperial Truth, which basically goes "there are no gods and you shall not worship anything, not even me - are you listening, Lorgar?!" As a result, no other Legion of the time HAD Chaplains, except for the Word Bearers (and Dark Angels and Blood Angels, to some extent); they were just being phased into place (as, essentially, Space Marine Commissars rather than warrior-priests) when the Nikea broke out to enforce the disbanding of librariums. Although most of them initially were Word Bearers chaplains sent to other legions (to secretly corrupt them through warrior lodges), most legions quickly started to train their own chaplains (mostly to get rid of the annoying outsiders), and they became adopted by both Loyalists and Traitors afterward. However, while loyalists found the Chaplains' ability to monitor and enforce loyalty to the big E invaluable, traitors weren't so fond of that particular role - after all, almost no one ever betrayed Chaos to the Emperor. Most the traitor legions' Chaplains became your average Chaos Lords or Sorcerers, but some particularly pious warbands kept them to maintain their religious rituals, while some freshly turned traitor chapters just kept their chaplains and converted them into apostles because they liked the idea of fanatical warrior-priests leading the charge while screaming litanies of HATRED at the top of their lungs. This change is why Dark Apostles went from a Word Bearers-exclusive unit to something any Chaos Marine army can field.