Heroes timeline.

The game tells the story of Vikings. So, it's clearly not part of the M&M universe.

Similarly, King's Bounty also has no plot connection to the M&M universe. Both King's Bounty and Hammer of the Gods are games from NWC that preceded Heroes, and they share some gameplay similarities.
It is generally accepted that King's Bounty was the technical, rather than the plot-based, ancestor of Heroes (the game was created and released by NWC). However, after playing Hammer of the Gods, you can see that this game is more similar to Heroes (one nuance - NWC did not create this game, but only published it).