Ultima Prima is an unobtainable vehicle in Just Cause Unity.
It is a giant mecha owned by Pablo Ordenado, it is used in the final boss fight if Murasame has been obtained. It is a giant, imperial looking mecha that stands on two feet. It is weaponized with laser and limbs the size of buildings.
(Performance was not good enough of a term to describe this things power.)
It has city-destroying capability, so is fought in the mountains of Gran Budgaria. It does have a weakness though, to make it semi-winnable.