I'm looking for a manga where the main character and his party defeat the demon king, but he curses the hero so he is immortal and slowly turning into a monster. He has the ability to absorb everything and he also absorbs a magic/holy sword in the later chapters. He lives in a forest with one maid. The fake hero comes to kill him but fails. That's when the real hero gets the magic/holy sword. It's not a isekai.
As confirmed in the comments, this is Shindou Yuusha to Maid Onee-san.
After defeating the Demon King and saving the world, Shion was exiled to the border. Here, the strongest Hero enjoyed his slow life with 4 capable onee-san maids.
Synopsis covers much of the question, except there are four maids instead of just one.