A'ce

A hundred years ago, an alien civilization arrived on Earth. They were called Aratheans. Those powerful being had technology that humanity could only dream of, if only it wasn't to be used against us... What they called Replacers were their main weapons: Mechanical creatures that are commonly called Tin Cans. They left our planet in ruins after stealing all of it ressources, transforming Earth into a sandy wasteland, only leaving behind Replacers to finish the job. The only way humanity have found to survive is using the energy left behind by Aratheans: Ether. This blue liquid evaporate at ambient, but can be used for batteries, ammunition and, under some circumstances, transform matter itself. Just don't breath it if you don't want to liquifie your lungs.