"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Ardal was the most respected and influential of the Bloody Baron's militia while at Crow's Perch. He served alongside the "baron" in the war, but after their group was massacred by Nilfgaardian soldiers, Phillip, Ardal, and the remnants of the Temerian forces in Velen took over Crow's Perch and ruled over the region, and though they maintained order, the Baron and his men harassed and brutalized the locals.
Around this time, Ardal also became the champion of Velen's fist-fighting ring.
If Return to Crookback Bog is completed: after the Baron left or hanged himself, Ardal took over, beginning an even harsher reign of terror.