Warning: The following text contains spoilers relating to the plot of the Factions Campaign.
The Juggernauts are Kurzicks who have sacrificed their humanity in order to become powerful defenders of the Echovald Forest and the Kurzick people. They have been transformed into large plant creatures by the Awakening Ceremony performed by the Tree Singers in the Eternal Grove, using the Forever Trees, Amber and Jade. As long as its Forever Tree lives, a Juggernaut is effectively immortal, as the destruction of a Juggernaut's body causes it to be reborn from its tree. This makes them fearless in battle. Becoming a Juggernaut is seen as the ultimate sacrifice and those who become Juggernauts are revered as saints.
For a list of Juggernauts found in the game, see Juggernauts.