Hellfire Engine (MToF)
The Hellfire Engine is a devilish construct, designed to mow down the hordes of any invading army wanting to take on the Nine Hells. They are a huge construct (making them immune to any anti-devil magics such armies tend to have), and apparently have some semblance of desire, seeing as how they occasionally get loose and go on a destructive rampage.
Hellfire Engines aren’t exactly capable of the subtlety or trickery employed by most devils, but archdevils keep them around specifically because they are so un-devil-like, and they are really good at dealing with armies. As an added bonus, any mortal creature killed by the Hellfire Engine becomes a new member of a Devil’s legion in a few hours. With this level of efficiency, you have to wonder why they don’t just take these out and invade the Material Plane on the regular.
In combat, the Hellfire Engine has a few options. It can either use its Flesh-Crushing Stride to pierce and break formations, or use one of its Hellfire Weapons to launch AoE Attacks on the enemy. A Hellfire Weapon comes equipped with a burning acid flamethrower (for damage over time and a cone attack), a lighting flail as a weapon attack to hurt targets with high saving throws, and a thunder cannon for ranged AoE coverage. All of its attacks have some kind of elemental damage, but none of the attacks share the same element, making it hard to shut down by simply using a damage resistance. Realistically, the Hellfire Engine has no true weakness, aside from flying enemies at extreme range (which is a weakness of nearly everyone). And it is very good at its combat role against hordes of lower CR creatures.
In the changeover, the Hellfire Engine went from +11 to hit, to +10 to hit on its Lighting flail (which was presumably a typo in the previous book, because the math didn’t add up to +11 either way). It also gained the Unusual Nature ability as befitting a construct.