The Demonlord is a boss enemy on the 9th floor of the Labyrinth of Chaos. There is only a single stage on the 9th floor so make sure you have the correct gear equipped when you start it.
It's not a very hard battle but much longer than previous bosses. Demonic and undead damage modifiers on your equipment would help tremendously. Take the life point prayer so you have more lives and this battle should be a breeze.
I would recommend level 90+.