Might you be referring to stories about a god, rather than a demon?
In the book "Time Patrol" by Paul Anderson, there is a story called "The Sorrow of Odin the Goth". In that story, there is a historian who is traveling back in time looking for local stories about Odin, because the tales told in that location deviate from the usual stories in interesting ways.
Of course, he first arrives well before the shift in stories, and sets up a backstory so he can be a frequent visitor. Over time, he becomes more and more involved in the locals' lives, bends a rule or two to give an advantage to his people, and the locals in turn pick up that he isn't, well, a normal wanderer - and Odin was known as a wanderer, who sometimes came in disguise, so clearly they believed that their occasional visitor was the god himself (and shaped their stories in response), even though he had never identified himself as such.
In the end, the historian is made to realize that the shift in Odin stories was the result of his own presence - tragically so.
the stories recite that the family that took him in were on the verge of victory in battle, when Odin suddenly appeared and betrayed them - warning their enemies of an attack and leaving the men fighting to their deaths.
His meddling had altered (would have altered?) the timeline enough that the outcome of that action would be different than was recorded, with resultant aftereffects, altering his own history in a way time travel could not sustain (they're fine with closed time loops, just not open changes). Instead, he has to fulfill the history as laid out to keep history on its proper track
he betrays them, saves their enemy, and puts time back on its proper track. When pressed for an explanation, he replies that "against time the gods themselves are powerless".
As for your comment, the series does indeed have extraterrestrials in it, and the time travelers are supposed to check if they were meddling in time. It also has time ships, and offhand I think they are described as egg-shaped, though I couldn't swear to it.