Bioterror is a secret boss in Raid. He is also listed as one of the strongest/hardest bosses due to his size, attack power, and defense.
Bioterror is a large mechanical skeleton the spawns in Hell. He is made almost entirely out of pure silver and some parts are made of only electric magic. He shares some attributes to the Icon of Sin from the DOOM series. This is proven by his large structure and his affiliation with the Underworld army.
In order to fight Bioterror, you must complete all 4 stories, defeat all bosses (not including secret bosses), and enter Hell once again. In the center of Hell where the Demon Hellord is supposed to spawn lies a mechanical skull, throw it into the lava, and Bioterror will spawn.
Claw: His standard attack where he tries to claw you.
Laser Shot: He fires lasers into the battlefield.
Pound: He pounds the ground so hard that all players will be launched into the air and take around 5-15 fall damage.
Fire Breath: Breathes fire...
Lava Wave: Summons a wave of lava.