Later on in the series, close to the end, the main character wins a trip from a slots event that sends him hundreds of years back in time. In that time period, he and the Eleanor fight an insanely powerful fighter using that time's Eleanor. He realizes that his Eleanor is far weaker than the one from the past. When he ends up talking with her, he tells her his story. She ends up having sex with him, then gives a massive chunk of her own power to the future sword explaining why his Eleanor had been weaker. The past Eleanor then blocked off her own memory of him and the events surrounding him so everything would happen the way it was supposed to. So the past Eleanor, now pregnant, ended up getting sealed in the cave that the mc found her in. And the child required a massive amount of the father's energy to be 'born'. That power was supplied when he burned off his massive arousal with raw force in the previous chapters, lol.
Edit: I forgot to mention that the power the past sword gave the present sword allowed her to gain a human form. That's how he got her pregnant in the first place.