A Warband is a loose Chaos Space Marine unit that somewhat corresponds with a Chapter of Space Marines. After the Horus Heresy failed 10,000 years previously, the Traitor Legions escaped into the Eye of Terror and retained the old Legion structure, while the Loyalist Legions were reformed into smaller Chapters. Thus, the original Legions of legend, founded by the direct hand of the Emperor, still exist among Chaos Space Marines. In actuality, due to the fractious and powerhungry nature of the servants of Chaos, the Legions have for the most part dissolved into smaller Warbands centred around a Champion who is able to bind them together to fulfill his objectives. The Legions themselves thus remain mostly symbolic, and in many cases, philosophical associations whose members share a particular outlook or mentality, or at least recognise a shared history.
There are some notable exceptions to the fractious tendency of the original Legions. The Iron Warriors Legion has remained particularly unitary and pragmatic, and has instead reorganised into Grand Companies, much larger and more stable than a typical warband, whose discipline and organisation allow the Company to continue to carry out the sophisticated large-scale manoeuvres for which the Legion is famous.
At the polar opposite to this lie the Emperor's Children and World Eaters Legions - whose extreme hedonistic megalomania, or mindless bloodthirst, respectively - prevent them from even forming coherent permanent warbands. Instead, they are found primarily as individual members of warbands made up of members of different Legions (or renegades), and are often viewed in their warbands as particularly deranged and psychotic berserker-sadists to be unleashed upon the enemy rather than fought alongside.
Aside from members who can claim origins to various different Traitor Legions, these Warbands of the later eras may also contain - or be composed entirely of - renegade Space Marines who have fallen from grace and turned to Chaos in the ten long millenia since the Horus Heresy, a fact the Imperium does not wish to be well known. These later renegades have turned traitor sometimes as individuals, sometimes as squads or companies, and in rare cases, entire latter-Founding Chapters have rebelled. Notable examples are the Red Corsairs and Crimson Slaughter .