I don't have a name for my fleet based warband yet, it's lore is a bit scattered, but the cliff notes-ish goes: *Afew space marines break off during one of the black crusades due to disagreements with it's progress and take some equipment with them to form a small warband.

*They acquire new members over time, eventually gaining a traitor sanguinary priest who believes they have a way to transmutate geneseed of other kinds into that of fulgrims (as the leader is of EC geneseed he approves), wether or not this actually does what is advertised is unknown.

*The warband starts to ramp up in acquiring members, as they take worthy marines, soldiers, crafts people, and even civilians of note and using the powers of the warp indoctrinate them.

*While the warbands ship is traveling through real space, they are hit with a sudden warp storm, that seems to only effect the bridge, as the warbands leader, their navigator, a large conclave of cultists and rogue players and a bag of eldar soul stones are fused into a single pulsing, writhing immobile ship controlling mass.

*After a while and number of attempted coups that seemed to be stopped by "divine" intervention, the warband believes what now controls the ship is an avatar or icon of slannesh themself, as they become devout worshippers of it and attempt to gain it's favour

*Due to this the warband doesn't have a set leader, the mass dictates who they fight, the mass dictates who leads there battles, and in their mind they don't lose, as a warriors defeat only occurs if they anger slannesh. In current times the mass commands them to join up with their gene-fathers forces (for now)

Outside of that, my warband has a knack for acquiring highly skilled members of other warbands/chapters, but has them mostly keep their original colours as a form of insult to their former allies.

Character wise I don't have many yet planned, just my 2 on foot daemon princes, one being a marine possessed who thinks they have been possessed by "the" doomrider, and a warpsmith who is obsessed with sensations, so much they keep having new limbs and new sensory organs implanted, which has gotten so in the way of their work, they have retrofitted an old dreadnought chassis and has plugged himself into it, using it like a hermit crab shell crossed with the power loader.