These murderous machines are a force to be reckoned with in the Dragon Quest video game series. Introduced in Dragon Quest VI as stronger versions of the Killing Machine series and usually the protectors of ruins and treasures. It's not known who built them or who are they working for.
They carry a sword and a mace for battle and are also able to shoot arrows with the crossbow on their "tail". One advantage is that they don't know many magic spells, they tend to attack twice though.
Dragon Quest VI
They appear at Pegasus Pillar as some of the strongest enemies in that dungeon, this enemy is able to attack twice per turn.
Dragon Quest Monsters Joker 2
An Overkilling Machine appears as the mid-boss of Bemusoleum and is avaliable to recruit if a Dragon Quest VI game is linked.
Its japanese name, Killer Machine 2, suggest it may be an upgraded version of the Killing Machines.