The strongest hero.

As I mentioned in my first post, the Overlord seems to be the strongest; he is, shall we say, "a class above" the others.
But it's not always the case that the strong defeat the weak; very often, the opposite happens, and there are plenty of examples of this in both life and in the game.
And it's not clear by what criteria to determine who is stronger: a one-on-one fight? commanding an army? maybe someone will out-magic someone else? under what conditions will the duel take place?

How to compare? Compare by all criteria and see who is stronger. According to the scenario, the Overlord is clearly the strongest; in terms of commanding an army (meaning defense, attack, and all bonuses to troops), only Markel (in the third mission of the sixth campaign), Gotai (after capturing Shahibdia, he receives large bonuses), and Biara can compete with the Overlord.
In terms of who will out-magic whom, it is clearly either the Overlord or Markel.
One-on-one - the Overlord will definitely win.

And Railag is indeed one of the shapers of Ashan's destiny. Besides killing Nicholas, this can also include the unification of Igg-Shail and the defeat of the demon worshipers there, and thus the participation of all Warlocks in the alliance at the end of the original campaign. In addition, Railag is the heir of Tieru and the only Dragon Knight (as far as I understand), thus he inherited the thousand-year history, power, and knowledge of this order, which already makes him one of the main shapers of Ashan's destiny.

He is a shaper of the destiny of Igg-Shail and (partially) Isabel, but not of all of Ashan. He could not eradicate the demon worshipers, and he united Igg-Shail for a short time. His participation in the campaign to Shio did not yield any tangible results for Ashan, and the results of the campaign worsened the already difficult situation of their world.
The fact that he is the heir of Tieru is, of course, commendable, but it does not play a decisive role in the fate of Ashan. Tieru fought the demons for 200 years, and only the last 20 years were somewhat fruitful. But he died, and note that not without the participation of the Overlord. And his heir could not stop Kha-Beleth - the Messiah is born, Ashan is in ruins.