Nightmare Engine

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

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, forcing an Autobot intervention. Ultra Magnus sent the Powerdashers to try and disable it, The Gloaming while Ironfist took his second life in the fight. Last Stand of the Wreckers #4

Kup was the last Autobot left inside the Nightmare Engine after everyone else woke up. Burning Bright

Ferak of Squadron X preferred to work on their ship on his Nightmare Engines. Tornado - Decepticon Saboteur