Will there be a sequel?

Perhaps he started helping the barbarians rise up and regain their former strength.

That's right :) he was just helping Verjak.

Here are some ideas I have:

1 - The time between the last chronicle and the campaign in Heroes 4.

There was a big battle during the Reckoning... between the barbarian (I forgot his name) and Jhelu, we have about 2 months in reserve, maybe he even participated in this war ;) (he was a general on one side... or maybe the echoes of the war affected him in some way... but he wasn't in the epicenter, otherwise he wouldn't have been able to escape).
And the last mission could be about how he ended up on Aksel and met Verjak.

2 About the necromancers... hmm, it's better this way... he's already immortal, he's probably bored... so he decided to turn to the necromancers... to put it mildly, to put his body and soul to rest (in general, to help him in any way they can)... and they decided to study him (deceptively), in general, he found out about this later, and then the necromancers were doomed :)
This is a brief outline, and it needs to be expanded into 5 missions with a cool plot