The Ash Warriors' training has, according to the Ash Warriors, been passed down since Luthias the Dwarfson first harnessed the battle-rage of the dwarves. Ash Warriors harness the battle rage inside, nurture it, and draw it out so they cannot fall in battle until their last foe is slain. Ash Warriors trust their hounds with their lives, as the hounds do them, and fight side by side with their hounds in battle.
In Dragon Age: The Calling, Kell reveals that he trained with the Ash Warriors. Part of their training focuses on the belief that you should "die" before battle (somewhat similar to the Legion of the Dead, a dwarven unit); this death is a ritual that, for Kell, involved the other warriors drawing blood from him via shallow cuts, and then rubbing salt into the bleeding gashes. Another similarity to the Legion is that the Ash Warriors' ranks are often composed of former criminals, joining the group as a means to redeem themselves. Their past identities are expunged, as "[e]ven in legal records their old life is erased—contracts voided, marriages annulled, records of birth erased."
Some time before or during the Blessed Age, the Ash Warriors were led by a man named Gelgenig who received a vision of Andraste while the Alamarri warred with the Avvars and Chasind over land. He then helped spread the faith of the Maker and helped unify the religion where Andraste could not, and ever since the Ash Warriors have been fierce adherents. However, this is not to say they are a part of the Chantry, as they predated its formation and do not practice the Chant of Light.
Ash Warriors are proud of their traditions; if the Warden in Dragon Age: Origins is an elf they will show little to no respect for you, if you are a human they will simply ask you to let them be, if however you choose to be a dwarf their leader will show you great respect and be much more polite.
They are found in Ostagar before the battle.