Mercury remembered himself to have lead the life of a mercenary until being recruited by the Founder to join the X-Men. The rest of this life remains in deep shadow.
In reality Mercury is a nanotech being created by Cerebro in order to aid it in accomplishing its "dream" of the capture, categorization and storage of every last mutant.
Cerebro's X-Men succeeded in kidnapping Shadowcat and defeating the real X-Men in battle. The team failed to steal a mutant tracking satellite due to interference from the real X-Men.
Mercury and the rest of Cerebro's X-Men were all absorbed into Cerebro itself and destroyed in order to grant Cerebro enough strength to try and defeat the real X-Men.
