In version 1.5, things were completely different for me. I quickly made my way to the prison and freed Godrick. And then something strange started happening – Faidaen appeared immediately, alone, near the last garrison leading to Markel's castle, with a decent-sized army – 20 treants, 20 horses, 30 druids... And he himself was a killing machine with attack and defense around 30, with luck, attack, defense, logistics, and siege engines with enchanted ballistae all upgraded. So, while Zehir and Godrick were climbing out of the dungeon, he simply took over all the castles except for the northwest one, leaving it for Godrick as a final challenge. By that time, the enemy's forces were simply laughable (by Faidaen's standards). He lost 10 horses, 5 druids, and all the dryads while conquering 4 castles. He could have lost even fewer, but after getting Faidaen, it didn't matter anymore.
Actually, he appears at a very convenient time because he has a pretty good squad for solving local tasks in the south of the map. And he appears as others mentioned above – after visiting the Red Tree, a little south of the fork in the roads. But in your case, did he appear without this trigger?
P.S. I finished the game, but it's interesting: Markel was extremely well-leveled in his mission, and I didn't know that I would have to fight him later. But in this mission, in the final battle, he turned out to be quite weak and with a not-so-large army. And everyone easily defeated him. As a pure fighter, Faidaen was the strongest of the three for me.