"The hour approaches. Our forces occupy the ring. Within hours, it will be under our control. Humanity will burn. Their brazen defiance will be all but a memory. No more Prophets, no more lies! We stand together, brothers to the end. We are his will. We are his legacy. We are the Banished!"
- – Warlord Escharum, Hand of Atriox
The Banished are a postwar Covenant splinter faction created during the Human-Covenant War, and one of the few universally praised additions to the Halo Universe that 343 (competitive) Industries has made to the Halo Universe. Combine Caesars Legion, the Covenant, and monke, and you have the Banished. Masters of Gorilla Warfare, the Banished are basically the Orkz if they had 2 functioning brain cells, but still didn't know how to use them.
A Brute-dominated Covenant rebel group started by Atriox, a Brute Chieftain who was done with the Covenants shit, and not being respected as the epic monke he is. Slavery and unwilling members dont exist in the Banished, as Atriox only lets people who want to join the Banished actualy join them (Grunts notwithstanding), as it was being forced into wars he did not care about that caused him to rebel from the Covenant. The Banished work as Mercenaries, and actually get paid for killing people, instead of just being told to believe in some "Great Journey" and that being their pay.
Atriox gladly works alongside a virgin Elite Shipmaster, Let 'Volir, because Atriox considers the Brute Vs. Elite Race-War a pointless waste of time, BUT, he is still totally ok with wiping out peaceful Human scientists on the Ark because, even in a pragmatic Mercenary militia that managed to survive the Covenant, the Xeno anti-Human hate-boner still reigns supreme.
The Banished were able to effortlessly outlive the Covenant, a vast, multi-species, advanced version of the Imperium of Man, but when they try to take over the Ark and use it as a superweapon to ransom the Galaxy into their control, a single Human Warship known as the UNSC Spirit of Fire that has been drifting in space for 23 years AND has half of its crew killed by Flood and Covvies back in the day gives the Banished a run for their money, and they are able to curbstomp them to the point that the Humans managed to create a new HALO and intend to summon UNSC reinforcements to it. (Assuming Cortana doesn't fuck up that plan, like everything else).
Meanwhile, back on the Ark, two Brute battle-brothers named Pavium and Voridus were reckless enough to release the Flood from the contained wreckage of High-Charity, the once flood-infested capital ship of the Covenant (Master Chief blew that thing up, killing most of the flood there, but whatever). Though, to be fair, Voridus was already established as a reckless warrior who brazenly dismissed the "myth" of the Flood against his brother's counsel. So not only is the Banished struggling against a UNSC ship they probably could have negotiated with if they resisted their Human hate-boner, they now have to deal with the Flood on the Ark.
Much later, the Banished somehow managed to take out many other UNSC ships and made their way to Zeta Halo, also known as Installation 07. A Banished Brute soldier known as "Craig" was also killed by Master Chief there, but trust me, he wanted a way out.
(additional story info will be written here once Halo Infinite is released)
As a mercenary organization, the Banished are essentially a military hierarchy led by Atriox. Their prevailing philosophy is one of ruthless pragmatism, with their operations focusing solely on accruing useful resources for their war machine. They are basically trying to gain control of the Galaxy with force, force, occasional subtlety, and more force. During the Covenant's reign, the Banished conveniently stayed quiet, so that way, 343 could explain how they just started appearing out of nowhere. Killing Covenant was never Atriox's ultimate target, instead only being leveraged as a source of supplies. It is possible that Atriox was planning to stabilize the Brutes on their homeworld Doisac under his leadership. However, it is suspected that Atriox's long-term goals were even more ambitious.
The Banished basically take whatever the Covenant, UNSC and Forerunners left behind, bolt some ugly metal plates over it, paint it red and point them in the direction of their enemies.
Societal conditions within the Banished reflect their harsh values, and most of the organization's members are mercenaries and killers. Due to the Brutes being screwed over by religion in the Covenant, the Banished are pretty much athiest, and their only desire is for battle and power. However, some individuals (namely Decimus) secretly harbor a reverence for the Forerunners.
Atriox claims he has no animosity towards Humanity, yet he still wiped out a peaceful colony of Human researchers on the Ark, that he probably could have easily incorporated if he reined in his soldiers. Atriox admitted respect for his enemies' tenacity and the fact that they were both survivors to the point that he offered the humans a chance to leave and in return, they would not be hunted down. However, Captain James Cutter refused the offer, because he is a badass.
The Banished have the potential to become something truly memorable, but Halo Infinite will truly prove if they have it in them or not. Regardless, they are infinitely better than Cortana's Skynet Uprising.