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So, way before there was a similar thread. It died off rather quickly so I would like to try again. In my opinion, it is always great to share the fruits of your imagination with others who may appreciate it and read what others have to say.
The Thread is not limited to any particular faction so feel free to post your homebrew: Planets, systems, campaigns, Space Marine Chapters, Imperial Guard Regiments, Eldar Craftworlds/ Exodite worlds/ Kabals, Ork Warbands, Tyranids hive-fleets/Genestealer cults, Chaos warbands and anything else I forgot or purposefully didn't mention.

Here is my piece of homebrew from the last thread.
This isn't exactly a chapter or a Warband, but a ragtag contingent formed due to one of the many warp-related scenarios in the 40k universe.

46th Holy Crusade to the Eye of Terror.

Chaplain Gerber Augustus 'The Fuhrer'

10th Crusader Squad
Sword Brethren Madalard
4 battle brothers ++Names expunged as unimportant by His Holy Inquisition, for further information address Black Templar records of The Cadian Crusade.++
4 assigned neophytes ++Names expunged as unimportant by His Holy Inquisition, for further information address Black Templar records of The Cadian Crusade.++

3rd Crusader Squad
Sword Brethren Valldemar
4 battle brothers ++Names expunged as unimportant by His Holy Inquisition, for further information address Black Templar records of The Cadian Crusade.++
4 assigned neophytes ++Names expunged as unimportant by His Holy Inquisition, for further information address Black Templar records of The Cadian Crusade.++

4th Sword Brethren Squad
Sword Brethren Castellan Garan
Sword Brethren Ingelbert
Sword Brethren Garvyn
Sword Brethren Sellig 'The Holy'
Sword Brethren Zigneuw 'Fury'

Stationed at ++All records destroyed, consult Black Templar records of The Cadian Crusade.++

The three Black Templar Squads were stationed on a dwarf planet named Ce17-0067 by the Cadian high command. The Crusade was tasked with up keeping communications and keeping an on ground orbital defence platform of thirty macro cannons. When Cadia fell in the wake of the 13th Black Crusade the 46th crusade was left with no communications and no reinforcements. Soon to make the matters worse a demonic infestation emerged in the lower levels of the facility. The fighting between the 46th crusade and Gurglon's plague host was fierce (any measurement of time would be useless as the fighting took place in a warp influenced sector.) although suffering multiple casualties the demon host was banished after Castellan Garan slew Gurglon in personal combat. When the herald was banished back into the warp the warp storm dispersed. After another week of standard Terran time, madness started to touch the minds of the Astartes. Three weeks after the Herald's death a Black Templar ship entered the vicinity and hailed the outpost.

The space marines on the ship were Primaris Marines of the Black Templar Chapter, namely the 10th and 4th Tactical Squads of the failed and reinitiated 46th Crusade formerly headed to the Eye of Terror as well as an additional squad of Sword Brethren.
The Brothers of the 46th 'failed' crusade were very visibly disappointed and angered by the Imperium simply casting them aside and replacing them with the better and improved copies. In following week of reparations done by the techpriests now assigned to the 46th crusade. An Iron Warrior ship The Steel Soul made planetfall immediately digging in.
Driven to the brink of madness the Sword Brethren Garan and Zigneuw crossed the dwarf planet and contacted the Iron Warriors.

It has been five days since the Sword Brethren left the orbital defence platform. Intercessor Gerham overlooked the wasteland from the bastion when he saw the two Sword Brethren returning. A bit surprised he allowed them to climb the battlements and asked them why did they leave. In response, Zigneuw grinned and swung his bolter crushing Gerham's coms piece. Immediately Garan powered his lightning claw and ripped through Gerham's chest, it didn't kill him but left him crippled and unable to fight back. Behind him, brother Siegbreu emerged onto the battlements and instead of helping him pierced one of his hearts with a combat blade. At that moment the Belisaurius Furnace activated allowing Gerham to throw Siegbreu off him only to be stabbed by Zigneuw in his secondary heart.

Chram looked upon the scene from afar with his cybernetic eye. At first, he was suspicious of the two marines especially them being the heir of Dorn, now he was sure of their loyalty and ordered his men to advance.

That night the Onground Orbital Defence Platform was filled with screams and gunfire as the 4th legion and the 46th Crusade slaughtered the Primaris marine contingent and the Chaplain. Then scavenging everything they could from the facility they fled into the Eye of Terror to Medrengard.

Here is the homebrew fluff for my Iron Warriors/renegades warband. Hope you like it and that I contributed to the discussion, looking forward to more fluff posts.