Void Stalkers

The Void Stalkers are a renegade Astartes Chapter and Chaos Warband lead by Diablo the Defiler. The Warband serves Azazel, a Greater Daemon of Chaos. Operating from their main fortress of Basilica Noctis on the planet Malleum in the Eye of Terror, the Void Stalkers terrorize the Galaxy by launching multiple raids throughout the Materium.

Angels Vigilant
The Void Stalkers began as a loyalist Astartes known as the Angels Vigilant and hailed from a temperate world known as Mavros in an isolated part of the Imperium.

The majority of the conflicts the Angels Vigilant fought were against Xenos and rarely fought against Daemons or Heretics. During the rare circumstance they did fight Daemons and Heretics, one of the Chapter's Librarians, Elymas, managed to acquire the chief artifact of the enemy Chaos Cult. The relic began to whisper to Elymas and convinced the young Librarian to keep it with him. Throughout Elymas' life and during his ascension to Chief Librarian of the Angels Vigilant, he continued to commune with the Daemon within the artifact.

Heresy
Roughly a decade prior to 236.M39, the Chapter's home sector was faced with the biggest Xenos invasion in it's history. The Sector Battlefleet and Militarum were eventually swept away, many of the populated planets where destroyed, and soon the Xenos host had arrived at Mavros. Because the Sector was in an isolated and unimportant part of the Galaxy, the Sector's call for help was ignored by the wider Imperium. The Angels Vigilant were left to stand alone against the impending Xenos invasion of their homeworldl

As the Xenos army made planetfall and swept through the Mavros country and the Angels Vigilant vanguard forces began to rapidly succumb to the Black Rage, the Daemon artifact once again called out to Elymas. This time, the Daemon demanded that the Chief Librarian take him to Hadrian Nero, the Chapter Master of the Angels Vigilant. The Daemon revealed himself to be a Greater Daemon of Chaos known as Azazel, and promised to save the Angels Vigilant from annihilation in return for the Chapter turning their back on the Emperor and serving the Four Ruinous Powers. With much of the Sector already destroyed and half his Chapter either dying in battle or succumbing to the Black Rage, the Chapter Master saw no reason to reject the Daemons offer. The Imperium had abandoned it and so the Angels Vigilant, now known as the Void Stalkers, would abandon the Imperium.

And so the Angels Vigilant, imbued with the diabolical powers of the Warp, began to push back the Xenos with savage fury. The battle was finally won when "Arch-sorcerer" Elymas freed Azazel from the relic and summoned him into Realspace. Together with their Daemonic master, the Angels Vigilant were able to slaughter the Xenos race to extinction.

Destruction of Mavros
Once the Xenos threat was eradicated, they immediately set to work carrying out the will of the Dark Gods. They renamed themselves the 'Void Stalkers', painted their armour purple, took full control of Mavros, and enslaved the population of their homeworld. They used their people as sacrifices, creation of new Astartes to rebuild the Chapter, form a Slave Army, and construct various temples, shrines, altars, effigies, etc. to the Chaos Gods. The new puppet government of Mavros officially succeeded from the Imperium, who finally took notice of the now destroyed Sector. They reached out to the Angels Vigilant and received no answer. A strike-force consisting of numerous Astartes Chapters lead by the Iron Spears were sent to the planet to investigate the situation. Upon arriving the Astartes discovered works dedicated to different Chaos Gods and the Chaos Cult that built them. As the Loyalists began to besiege the planet, not only did they discover that corruption ran rampant throughout the entire society, but they were assaulted by a frothing unit Death Company Marines dedicated to Khorne bearing the markings of the Angels Vigilant Chapter. This lead the Loyalists to the realization that the Angels Vigilant were responsible for the corruption, a realization was cemented when the Angels Vigilant  (now painted purple and called the Void Stalkers) exited the Fortress to engage the Loyalists. After a grueling battle with the Chaos Slave Army and the Void Stalkers, the Loyalist Force Commander ultimately declared Exterminatus on Mavros. While the Imperium believed that the Void Stalkers were destroyed along with Mavros, 300 Marines (with the help of Azazel) managed to escape to the Eye of Terror on a damaged Battle Barge.

Rebuilding the Void Stalkers
After taking refuge within the Eye of Terror Azazel lead the surviving Void Stalkers to his world Malleum, a volcanic Daemon/Death World within the Baleful Eye System. Orbitin Malleum was an artificial moon known as Donn, home to a Dark Mechanicus enclave. In return for a small share of the slaves and resources acquired by the Void Stalkers, the DarkMech agreed to faithfully serve the renegade Chapter. Next, the Void Stalkers began to commit small and covert raids on either small isolated Imperial worlds, worlds containing Chaos Cults, or Daemon Worlds in order to make an Army and a workforce as well as create new Heretic Astartes. Next the Void Stalkers set about making alliances with various Chaos Warbands within the Eye of Terror, particularly amongst the Word Bearers.

For the next 440 years the Void Stalkers rebuilt themselves, constructed great fortresses upon Malleum (including Basilica Noctis), pieced together a massive army of cultists and traitors, and with the help of the DarkMech of Donn, build numerous weapons, Daemon Engines, and a mighty Warfleet.

Battle of Ailinne
Once they had completely rebuilt themselves the Void Stalkers sought not only to make their presence known to the Galaxy, but also to create their own Empire. The Void Stalkers first target was Ailinne, homeworld of the Iron Spears, the Chapter that lead the assault on Mavros and ultimately destroyed their original homeworld. Together with several Warbands of the Word Bearers and World Eaters, as well as a large Cultist Army, several Congregations of the Void Stalkers lead a mighty assault on the frozen planet of Ailinne.

Initially, the battle was not in the favor of the Traitors, as the Iron Spears Fortress-Monastery, Dunbairrche, was armed with large defensive canons, almost completely surrounded by impassible mountains (with only one narrow protected pass), and protected above by void shields. The battle was turned in favor of the Traitors as Arch-sorcerer Elymas reached out to and corrupted Gofraid, Chieftain of the Iron Spears' 10th Clan-Company. Gofraid betrayed his brothers, shutting down automatic defenses and opening the Blackstair Pass. Although the Iron Spears were able to initially hold the gate, they were eventually overran by Khornate Berserkers. As Heretics pushed deeper into Dunbairrche, they found themselves unable to breach the inner sections of the Monastery. Eventually several Companies from the Blood Angels and Broken Wings arrived, heeding a distress call from Iron Spears Sergeant Murchadh, and pushed out the Traitors.

Although they were defeated, the Heretic Astartes were able to slay 62% of the Iron Spears Chapter, including the Chapter Master.

Organization
The Warband itself is divided into thirteen semi-autonomous groups known as "Congregations". The number of Chaos Marines in each Congregation varies wildly from group-to-group and time-to-time; but usually it is never bellow 100 Marines and never above 300 Marines. This makes the lowest number of "Void Marines" 1,300 and the highest number 3,900.

The ranks of the Void Stalkers are bolstered by innumerable Daemons from the Four Chaos Gods, a massive Cult/Slave Army, and an impressive Warfleet typically manned by Cults, Slaves, or Hereteks of Donn.

Cultists that serve the Void Stalkers in any capacity are considered to be a part of an organization known as the "Disciples of Malleum".

Mutations
All the Marines of the Void Stalkers display the same set of chaos mutations but to varying degrees. Physically the Void Stalkers are unnaturally pale (from sickly pale to chalk white), have abnormally colored irises (that sometimes glow), show heavy bags/shadows under the eyes, display pronounced canine fangs, and have an abnormal and disturbing voice.

Mentally, the entire Warband has been driven mad by the taint of the warm and its members can shift moods within an instant. Void Marines are as quick to anger as they are to jest. They can be impatient but can either show remarkable initiative in the face of the enemy or make utterly horrible suggestions that make sense only to their broken minds.

Tactics
The Void Stalkers typically make use of shock assaults alongside terror tactics.

Prior to a raid, the Void Stalkers will hijack the planet's vox network in order to announce who they are, why they are attacking the planet, and that the will arrive shortly. Afterwords they begin to broadcast their dark litanies, heretical propaganda, and most of all horrifying noises and imagery. All while striking fear into the hearts of those who dare to potentially depose them. The Void Stalkers are not subtle about deployment. Attempting to be as loud as possible. Their battle tactics are volatile and are prone to change on a moment-by-moment basis. The only constant is wanton murder, chaos, and destruction. The goals of their raids range from looting and conquest to utter annihilation and mindless slaughter.

Demeanor
The Warband holds the word of the Ruinous Powers above all and believe they will guide them in all things. They do not worship any one Chaos God but rather worship all four as a "Combined Pantheon". As stated before they preferring to make bold, frontal assaults and declare their allegiance immediately (sometimes before the battle even begins), so that the enemy knows to be afraid and that the servants of the Dark Gods' have come for their souls.

Chaos Marines of the Void Stalkers Warband bear a bitter hatred towards icons and relics of the enemy, and will try to destroy or corrupt them whenever the chance arises. Temples burn and works of art are twisted by their hands. Anything that is not corrupted or looted is obliterated. Some Chaos Marines become so addicted to this wanton destruction that they will place the chance to slaughter masses of innocents and enemies above all else. This can cause wider tactical problems, especially when working with other more coordinated Renegade Chapters or Chaos Warbands.