The aim of the VCMI project is to rewrite the entire Heroes III engine
(VCMI is NOT another mod) giving it new possibilities. Years of
intensive work resulted in creating application with impressive amount
of features. Among the current features are:

- Complete gameplay mechanics
- Almost all objects, abilities, spells and other content
- Basic battle AI and adventure AI
- Many GUI improvements: high resolutions, stack queue, creature window
- Advanced and easy mod support: add new towns, creatures, heroes,
  artifacts and spells without limits or conflicts
- Launcher for easy configuration: download mods from the server and
  install them immediatelly!
- Random map generator that supports objects added by mods

NOTE!
You need the original game data to use this.

HowTo:
You can find a step-by-step tutorial for setting up the game here:
https://slackalaxy.wordpress.com/2015/05/02/heroes-iii/