Monstructs are enemies from Hacx. They are huge and slow mutants which possess 400 hit points and are armed with two plasma cannons and a machine gun on their head.
While their health is somewhat low, their attacks are deadly. A monstruct will start firing two waves of plasma orbs at a target. These orbs are extremely fast, hard to dodge and do a lot of damage, to the point one of them can kill a player with 100% health. They also have a high splash damage. If anything managed to survive from its attack, the monstruct will start firing his machine gun at its target until they move out of sight. It is recommended to attack these mutants as quickly as possible to prevent them from attacking.
There is also a rare variant of this creature disguised as a statue. Shooting the statue will activate it. This version of the monstruct features notably more health than the normal version, but has a considerably higher pain chance. It's pain graphic though seems to be incorrect.
Also, note that the monstruct is actually not invulnerable to the splash damage of his own projectiles. If the projectiles explodes right near him, he will probably be killed in the explosion.