Demon executioners are level 97 combat Summoning familiars. They belong to the Demon group of familiars, and currently share the special move, Ring of fire, with two other demons: demon deacons and demon brawlers. It is the ranged damage variety of the group. This group of familiars also share the unique trait of being aggressive; they will automatically find and attack new enemies of the type you are currently attacking (though you must be in combat for their aggression to activate).
When in a God Wars Dungeon encampment, this familiar will treat non-slayer creatures as the same type of enemy, thus engaging any of them in combat automatically. If you attack a slayer creature (i.e. spiritual mage), however, they will begin to attack that class of monster too.