If it has a pulse,Take its skull!If it builds a house,Smash it flat!Strength is my God,The God of Shapes,If my God should fail me,I will kill him too.— Sword Law Mantra of the Knights Belligerent, Kill Six Billion Demons
Tell them ruin has come to their world, death, despair and red war...
Tell them their hopes and pride have come to nothing.
Tell them their empty whispers fall upon deaf ears - their gods are dead, human logic has killed them.
Tell them the Angels of Death have come.
Tell them that nothing can save them now.
— Perturabo, Primarch of the Iron Warriors, Warhammer 40,000
[Content warning: mass murder, self-harm, psychological abuse.]
Raiders are nasty, but they can be bargained with. They want your weapons, your supplies, your vehicle, and you can trade your life for these things. They also want to live, and you can trade your safety for their lives, should you be well-armed enough.
Warlords, likewise, are nasty, but can also be survived. They want control and obedience, and they care about stuff of the old world, like symbols and status. Grovel at their feet, or prove your usefulness, and the warlord might let you live.
The Ash Legion is not like any of these. These people genuinely, actually want to kill you.
When they come to your settlement, the Ash Legion make no demands and present no ultimatum. They lay siege to your town, burn everything to ashes, and kill or enslave everyone inside. Nobody is spared, no matter what. After this, they smear the ashes of your burned home on their armor. They beat the slaves until they stop wailing. Then they march off into the wasteland to find the next town to destroy. Pathetic scavengers flock in their wake, like hyenas following a lion, picking their massacres clean.
They don't take any more supplies than they need to keep going. They do not trade, and do not hoard riches. This is because the Ash Legion isn't interested in riches. They only ever take enough to keep going and leave the rest to the scavengers. They're not conquerors or raiders. They kill for killing's sake.
They have a philosophy; they call it the Truth. The Truth has two central tenets which informs the entire worldview of an Ash Legionnaire.
The Tenet of Strength: Only strength won't be eaten in the end.
The Tenet of Strength professes that external strength is the final measure of right and wrong. There is no such thing as a moral victory. There is no such thing as an unfair fight. Crushing your opponent is a self-justifying act - if he were superior to you, he could've crushed you instead. What values they may have had that could've flourished if you hadn't destroyed them are worthless - afterall, they didn't prevent them from being crushed by you.
A corollary of the Tenet of Strength is that anything that detracts from one's strength is a flaw that must be seared away. Strength is the natural state of man, and the only reason people are weak is that their flaws - comfort, compassion, cowardice - are denting their strength. Strength is not a state to be achieved, but the default state of things, which must be restored by a ruthless purge of all flaws and weaknesses. The ideal state of the Ash Legionnaire is an emotional wreck who burned all bridges in his life to attain the greatest capacity for destruction possible.
They are fully aware that this way of thinking is extremely psychologically unhealthy and that they're purposefully emotionally stunting themselves, but since they consider emotions to be a distraction from the ultimate goal of strength, they literally just do not care. They're absolutely miserable and they do not give a shit about it, as long as they can be strong.
The Tenet of Eternity: Only the irrevocable has value.
If you build a tower, it'll eventually collapse, but if a meteor strikes the earth, and wipes out twenty towns, the crater remains there forever. According to the Tenet of Eternity, no matter what you build, someone stronger will eventually tear it down and reduce it to irrelevance, so instead you should cut irreversible scars into the world to mark your eternal legacy. And of course, the purest kind of irreversible cutting is murder, since each human life is unique and irreplaceable.
Because impermanence is irrelevance, and the only way to actually achieve an eternal legacy is to destroy something irreplaceable, before someone stronger than you inevitably kills you in turn. As you have destroyed something irreplaceable, you are now the orchestrator of irrevocable change in the world, and thus you've achieved immortality.
The Ash Legion is fully aware they're trampling on countless good and innocent things. They're not dumb brutes or mindless berserkers. They're often actually quite intelligent, and very philosophically-inclined, and many of them keep slaves for the express purpose of debate. It's just that their philosophy dictates that razing towns to the ground and tossing their screaming populations into great burning pyres is the only worthwhile thing to do.
Ash Legionnaires wear power armor. They're legitimate sets of military power armor from the war that ended the world, plundered from buried weapon caches and military outposts. They're seen as the symbol of their cold, strong exterior, so the Legionnaires fight fully suited, with their nonfunctional HUD helmets obscuring their faces and presenting only a merciless mask of iron. Most of them consider the armor to reflect their inner self better than the weak, squishy meat body encased within.
These suits are miracle tech by wasteland standards, but they're almost all massively decayed and most of their functionality is gone because they cannot be properly repaired or maintained in the wasteland. Their servo actuators can barely move anymore, for example - the joints and electronics are so damaged that they can only muster a few minutes of powered movement, then they break down and must be repaired. Despite their high-tech origins, they tend to look partially scrapped-together as irreplaceable components break and are replaced with inferior parts scavenged from scrap. Most Legionnaires wear them as inanimated armor and use the power armor functions for short boosts of strength.
1d10 Ash Legion armor names: 1. The March, 2. Soul Acid, 3. Earthscar, 4. Adamant Skull, 5. Exhalation of Smoke, 6. Absolution, 7. The Pyre of God, 8. Eternal Tortoise, 9. Scorn, 10. Monolith of Truth.
The suits are passed down along master-apprentice chains, with each Legionnaire heriting his armor to his pupil in the art of hateful killing. Every once in a while, an Ash Legionnaire will take a young boy from a settlement he just razed, and make him his apprentice. The Legionnaire probably killed this boy's family with his own two hands.
The boy will hate the Legionnaire. The Legionnaire will encourage this. The boy will learn the truth of the two Tenets under the Legionnaire's hand, until such time that his hatred swells to such proportions that he finally tries to kill his master. If he is too weak, he will be killed instead; a failed apprentice. But if he is strong enough, the Legionnaire will die like the worthless, weak dog he is. The boy will peel his master's corpse out of his armor, and inherit the armor and its name. A new Ash Legionnaire is born.