The term Demon Lord came into usage in the Abyssal Release (7777 DE - 8777 DE). The primordials used it to refer to the commanders leading the scattered uprisings taking place across the worlds of Piranoths Steps.
When Piranoths Steps was created in the deepest reaches of Chaos, it is said that the evil emanating from this realm corrupted some of the mighty primordials. Demogorgon, Baphomet, and Orcus were some of the primordials that became infused with chaos and evil and then reshaped into the likeness of pure destructive evil. They became the demon lords, masters of entire worlds.
When we defeated the primordials, we opened the way for this demon invasion.
- Naraz-Nâru, from a mithril plaque at Ginbar-Karud - "One Darkness Unto Another"