I was once toying with the idea of a dark, evil dominated world. You know one that is on the verge of evil winning a total permanent victory.
Then I was going to have the remaining forces of good summon something... alien.
The hook was going to be that good and evil were totally irrelevant to it, but it was summoned to do a job, so it was going to be done. Really it wouldn't even care if evil won.
So basically the thing starts murdering every member of the evil forces it can, and it is very good at it. It takes no pleasure in doing it, it is basically a walking meatgrinder that lays waste to individuals and whole cities as it goes.
I was going to have it be intelligent enough to avoid any dangerous evil opponents, like overlords and whatnot. But it would decimate any easy targets it finds. It doesn't get scared, doesn't sleep, and never tires. It just kills and kills.
When you call it, it won't stop because you changed your mind. It will complete what it was summoned to do. And it is going to kill all the evil people, or at least forces in the world, the easiest first.
It won't attack your side unless you get in the way, but if you do it will deal with you like the others. It doesn't fear demons, undead, liches, anything like that. Those are simply tactical problems, and it is very capable. If it took 10,000 years to get them all it won't mind.
In short it would be a berserker (from the Saberhagen books), or perhaps like "Good? Bad? I'm the guy with the gun."