I don't know any other way to say it so I'll just go into what the first chapter is about:

The first page basically shows 2 people killed by some armed guards, they say some remark and then walk out of the door. Next page after it cuts to this old man digging away inside this cave. When he's digging out a wall, it collapses and reveals a room that has flowing mana (magic, whatever) in it. He absorbs some of the mana and rejuvenates himself and gets a younger look. After that's done, he proceeded to summon a demon to be his assistant, he does it and they have some sort of banter. She agrees and the contract is signed... And then he almost immediately proceeds to bang the demon, something about dumping excess mana into her (basically making her into his own personal battery pack). Guy says he wants to be a demon lord and then the chapter ends... Or maybe it ends after the sex, it's one of those 2. But yeah, I can't remember the name of the manga