From their website:
The helljack known as Erebus serves Lord Exhumator Scaverous, murdering all in its path as it eagerly gathers souls for its master. The lives Erebus has claimed leave it steeped in the detritus of a thousand deaths. This spectral effluvium envelops the machine, repelling any who try to strike its blackened carapace.
The Erebus warjack upgrade kit is not a complete model. Players will need a Cryx heavy warjack plastic kit (PIP 34066) in addition to this kit.