I don’t remember the name of the demon lord but he reincarnates as a young boy with purple(I think) hair and he is really overpowered. I remember him having a true form that looked kind of like a galaxy alien, and had some companions that remember him from before he reincarnated. One part I remember is he’s fighting around a giant sword and a city and has to protect his friends from a giant magic spell.
this is I, the Strongest Demon, Have Regained My Youth?!this is a manga where a galaxy looking entity who had built an empire and tens of thousands of people submit to him. To get more power he entered the realm of the demons and was looking for the mystery of all lives but in doing so he was gone for thousands of years and his empire is nothing but a market. His name is lauded the demon king