Legends of Tomorrow has revealed the identity of the evil force hounding John Constantine as The Demon Neron. "Hell No, Dolly!", season 4 episode 7, also divulged the scope of Neron's plans and firmly established him as the chief villain whom the Legends will have to defeat by the season's end.
It was confirmed in Legends' season 4 premiere "The Virgin Gary" that some supernatural force had it out for John Constantine. In "Hell No, Dolly!", John told teammates Charlie and Zari about Neron - a powerful demon who had tried to blackmail John into helping him overthrow the triumvirate of demons that rule Hell. John refused, despite Neron having acquired the deed to John's damned soul and having the authority to do all manner of horrible things to John after he died. The resulting conflict ultimately ended in Neron being banished from the material plane, but dragging John's boyfriend Desmond (who had sold his soul to Neron to buy John's safety) back to Hell with him.
First appearing in Underworld Unleashed #1 in 1995, Neron is one of the more infamous players in DC Comics' cosmology. A demon-lord who can't resist making Faustian bargains with mortals, Neron once tried to take over the universe by gathering a mass quantity of souls, which are the currency of Hell and the source of his power. Neron approached numerous supervillains then numerous superheroes, offering them their hearts' desire in exchange for their souls, all while plotting a way to acquire the purest soul on Earth - an object that would make Neron all-powerful! Though his scheme was ultimately thwarted, Neron has remained a power to be reckoned with in the DC Universe, always ready to make a deal with anyone desperate enough to seek his help and pay his price.
It's worth mentioning that John Constantine has battled Neron before in the comics. Indeed, Neron was one of the chief villains of the Constantine The Hellblazer series, which reimagined Neron as a demonic stockbroker. The events of the series' second major story arc, The Art Of The Deal, seem to have inspired a bit of Legends of Tomorrow's current storyline. Both involve a Neron who sought Constantine's assistance in staging a hostile takeover of Hell and both featured John losing his boyfriend to Neron's machinations, though John's boyfriend in the comics was named Oliver rather than Desmond.
It remains to be seen how closely Legends of Tomorrow will adhere to the story from The Art of The Deal. It seems unlikely, however, that we will see Neron giving a speech to his followers where he promises to "Make Hell Great Again" as in the comics. Still, it seems clear that with a power-minded Neron seeking to gather souls as part of a ploy to take over Hell, the Legends may have their hands full for the rest of season 4.