Titan

They are "sacred war machines" of the Adeptus Mechanicus. They fuck up your shit. They come in big, bigger, bigger than the last one, damn that's one big mecha and WHAT THE HELL IS THAT GIANT MONSTROCITY!

They also tend to fuck up your shit.

To elaborate: Titans are the biggest freaking land machines the Imperium has at it's disposal in the GRIM DARKNESS OF THE 41ST MILLENNIUM.

How big are they? Think a giant mecha the size of the Empire State Building.

How powerful? Take a couple of Yamato-class battleships and add More Dakka.

How scary? The apocalypse is walking towards you; say prayers to whatever diety you believe in and hope it uses the bolter cannon first, cause that flame thrower hurts.