OP here, I'm gonna share the back story so you guys know what you're dealing with.
The leader of the warband was never any part of a formal military structure, and never received any formal training, but he is naturally strong and accrued lots of battle experience in street skirmishes before he embraced Khorne. The warband was originally a large horde of rebels on a hive world that was ruled by a particularly tyrannical and decadent upper class, comprised of regular citizens who took up arms and traitor guardsmen. They were so enraged at the rulers and inflicted so much death and destruction that Khorne reached out to them. By the time the rebellion was over most of them were dead, the large majority of the survivors being traitor IG, and the remnants of the rebel mob formed a more organized group, scrounged up what equipment they could, stole some ships and took off to look for more "sinners" to "punish."
They see themselves as crusaders for justice, rather than simple warriors, and they bear a particularly ferocious hatred for anyone who is wealthy and powerful, and especially the followers of Slaanesh. They are quite young, having only existed for just 20 years, but their initially large size allowed them to do some damage for the first decade or so, with heavy losses. They are mainly pirates, preying on trade vessels, and whenever the opportunity presents itself they attack small and isolated Imperial Guard regiments for equipment, and of course skulls and blood.
I haven't yet decided how many ships or what kind they have yet, but I'm thinking their numbers will be in the high thousands and mostly traitor guardsmen. I also haven't decided how much and with what Khorne has blessed them yet, but their leader has definitely been beefed up quite a bit.