Lucius being the son of Maximus in Gladiator 2 may have come as a shock to some, but the original movie laid the foundation 24 years ago. Paul Mescal leads Gladiator 2’s cast as Lucius, the son of Lucilla (Connie Nielsen) from the first movie. Audiences catch up with Lucius fifteen years after the death of Maximus (Russell Crowe) in the Colosseum, and a flashback scene shows that he was forced to flee the city, as being the heir to the throne would make him a target. Now, he’s living in Numidia with a wife, tending to a farm.

https://screenrant.com/gladiator-2-movie-maximus-lucius-father-retcon-explainer/