I don't know how feasible this is:
When encountering a hostile hero, there is a chance (5% + 1% per 5 hero levels) that they will switch to your side.

I leveled up my hero, tried hard, honed my tactics – I simply cherished and nurtured my main hero, and then suddenly he becomes hostile! It's absurd!

If the main hero is killed, it means you've already lost (but even then, there's a chance to escape, to buy your way out). That's how most games are structured. But to lose without a battle is even more frustrating, and without any strategy – you just met and that's it... :smile30:

P.S. Technically, this is feasible :)