He has a past with Hal, knowing him for centuries when they cut a bloody path through Europe. He had two children when he was human, both of whom died of old age. He mentions he has living descendants with whom he checks in on from time to time, to "see how [his] genes are getting on".
Fergus was desperate to impress the Old Ones; he kidnapped baby Eve in the first episode of Series 4, hoping that after seeing him apprehend the War Child they would offer him a high-ranking position. When Eve was stolen back by the main cast (Annie, Tom, and George) Fergus began to stalk and torment them, threatening Annie and trying to return Hal to his murderous ways. Fergus turned four people into vampires, called them his strike team and led them on an assault on Tom's cafe. Later he led them to the house and tried once again to steal Eve, however his strike team were all killed by Tom and Annie, and Hal himself slayed Fergus.
Fergus and Cutler did not see eye to eye, probably due to their conflicting personalties. Cutler found Fergus's tactics thuggish and stupid, whilst Fergus thought Cutler was annoying and a wimp because he wouldn't resolve his problems through violence. In one of the flashbacks in episode 7, we are shown Hal comparing Cutler to Fergus (both vampires he has recruited) and implying that Fergus is a better vampire because Cutler refused to kill.