In battle, press F1 to display dialog with special terrains choice. Clicking on dices button will exit the dialog and a random battlefield will be selected. The KEY choice can be changed in Battlefields.ini (F1-F4). F1 is enabled by default.
The new battlefield will display in game as soon as your creatures moves, or your hero casts spell. If someone finds out how to enable it instantly, become my hero.
Warning: this mod allows cheating. If you change battlefields in custom maps, you may experience game-play dead ends or destroy challenges.