A supercombatant, or SC, is a heavily equipped unit that is powerful to start with and probably capable of casting his own self-buffs. These guys are traditionally capable of taking on armies. They are most viable when opponents field single-type armies, and become less viable as armies diversify. They can also be uneconomical if you build them on a template and your opponent is able to figure out a decent counter.
An SC is built on a limited and exclusive collection of Commander chassis. SCs are often given 50+ gems worth of equipment and should be able to take out entire armies with that equipment. An SC Chassis often, if not always, has more than 60 HP and 3 or more inherent forms of damage mitigation from abilities or magic paths. They are more vulnerable than they were in Dominions 3, so they must be carefully equipped and only used solo when you are certain that the opponent cannot counter it. SCs are commonly used alongside armies because many anti-SC strategies will fail if the opponent is unable to isolate the SC, and the tactics for killing an SC may be different from optimal tactics against an army.
Still, they can be demoralizing and very difficult to face, especially earlier in the game, and can become much harder to counter when added to an army.
See Building thugs and SCs on useful tools for them.