Mountain King is a Force affinity legendary from the Dwarves faction. Mountain King is the ultimate defensive Arena champion with extremely hard hits and high survivability. With extremely high base HP he can be built as a tank, with his damage scaling by the HP you are building, just make sure you build high crit rate and damage as well.
Dwarves also have a high base resistance and a high resist Mountain King can be really hard to kill.
Mountain King is also strong for Faction Wars especially because Dwarves lack many options for damage.