Towering above any player character, at about 20 feet tall, the behemoths are the greatest threat to humanity found among the Super mutants.
There are only five behemoths to be found in the game, although reports of their sightings continue through dialog amongst the Brotherhood of Steel. These super mutants are the oldest and strongest of their kind. As well as being many times larger than any other humanoid creature, for unknown reasons, they will not speak; instead they will make monstrous roars.
They are usually armed with an almost unrealistically large fire hydrant mounted on a large pipe; other than that, they also use their enormous fists. Either way, both weapons are heavy-hitters, and will cause major damage against any enemy smaller than them.
Killing all five behemoths will unlock the achievement/trophy, The Bigger They Are…