Play as the High Elves!!!
On chapter five of the Undead campaign, "Fall of Silvermoon", stop the runners and destroy the High Elf base on the upper region of the map. After you do this, you will be able to use the Banshee. Fight your way into the lower High Elf base, and use the Banshee's Possesion on a High Elf peasent. You can then use the peasant to build a High Elf base, allowing you to build all their units, and access all three human heroes.
More tricks, tips and cheats for this game are right here - Warcraft III: Reign of Chaos cheats
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