Nightmare Engine

The Nightmare Engine is presumably some sort of Decepticon weapon from the IDW portion of the Generation 1 continuity family.

We really don't know much about the Nightmare Engine.

Remember what Optimus said - 'That which is, isn't.'Ultra Magnus warns his troops about the Nightmare Engine, "The Gloaming"

Fiction

IDW Generation 1 continuity

Ferak designed the Nightmare Engine, an impressive feat that had Megatron acknowledge the engineer. Ferak's Squadron X profile

Megatron activated the Nightmare Engine on the planet Clemency; Ultra Magnus sent the Powerdashers to try and disable it. The Gloaming

Ironfist took his second life after Megatron activated the invention. Last Stand of the Wreckers #4

Kup was the last Autobot left inside the Nightmare Engine after everyone else woke up. Burning Bright: Dark Cybertron Chapter 8