'Heavy Metal' is the name given to a gestalt entity. The entity itself is created from four human friends and an extraterrestrial being known as Aurus'tak - made of living metal.
Arus'tak's species originally came from Sarophagas - the same world as Pulichina - and were in fact a sub species of the Metalloxis. However, the majority were wiped out when the Vartoks destroyed the planet (as they were much more resilient to mind control then the Metalloxis and as such still resisted when the Vartoks tried to capture them).
When the Vartocs invaded Sarophagas - intending to control the Metalloxis and any other species with potential use on the planet - they were initially repelled thanks to the abilities of the Metalloxis. However, it would not be for long as the Vartocs utilized heavy-duty technology in their attempt to subjugate the species. Surrounding the planet in an energy shield, the Vartoc's utilized a 'planetary-hypnosis engine' to make the memories of the Metalloxis dormant before implanting new memories to make the Metalloxis subservient to them.
The Vartocs then transported the species to one of their 'slavery worlds' before trying to capture other species on the planet that had use. One of these species was a sub species of the Metalloxis, but whom some members of their race had proven more resilient to the mind control. As a result, the Vartocs still faced some opposition - but it was now much easier to take control of the planet. For some Vartocs, though, it wasn't an easy ride. Aurus'tak - one of the species who had resisted the mind control - managed to hijack a Vartoc vessel and pilot it away from Sarophagas before absorbing the ship into himself - thus gaining its abilities.
(To be added)