Today, I had the same question as the original poster: "how to complete the 'Croatia' scenario."
Version Heroes of Might and Magic 3.5: In the Wake of Gods
Is this version from a different thread?
Unfortunately, the original version 3.0 didn't launch on my OS, so I couldn't check.
In version 3.5, repeatedly "drinking" from the magic well leads to random results, including a tunnel to another well.
After spending two months :D of game time, I finally made it to the dungeon.