During Chapter 5 of the Undead campaign, Fall Of Silvermoon, you can create an army of heroes (maximum is usually 3). You must complete the optional quest. First, destroy Sylvanas' base leaving at least one Human Peasant alive. After the outro sequence, you can build Banshees. Create one, and send her to the Mana shrine in the middle island so that she can quickly gain 250 Mana. Then, command her to take control of the Peasant. Build a Town Hall and upgrade it to a Castle. Then, build an Altar Of Kings. You may now build as many heroes as desired, as long as your upkeep can support it.