Welcome to Warcraft 3 Extended Edition, a project with the goal of expanding the existing game by adding unused features from the campagin as well as by adding new content of the highest possible quality while trying to keep the game as balanced as it can be.
To obtain 'The Power of Fell', the player needs to upgrade to Stronghold, build a Beastiery and research Berserker Strenght upgrade.
When researched it replaces the Far Seer and Tauren Chieftain with Shadow Warlock and Orc Warchief, while the Shadow Hunter in permanently lost. Also the Blademaster gets the model of the Blademaster of Blackrock Clan from the campagin.
Grunts, Raiders, Kodo Beasts and Peons turn into their Fel equivalents. Troll Headhunters and Berserkers are replaced with Amani Berserkers, which, like the Troll Berserkers, have Berserk ability. Also the Pigs turn into Fel Boars.
Shamans and Troll Witch Doctors are replaced with Warlocks, which have Firebolt instead of Purge, and Amani Shadow Priests, which initially have Ray of Disruption and can learn Acid Bomb and Sleep. Spirit Walkers and Tauren are lost.
Lastly, Wind Riders are replaced with Black Drakes, a heavy ranged flying unit with Keen Senses ability, which reveals nearby invisible units and can also be upgraded with Dragon Skin, which gives a chance to reduce incoming damage.
Note: Dragon Skin is analog to Envenomed Spears upgrade.