WsawW
The doom lord was bugged in 1.6c, cause the update logs aren't exactly very clear chances are Aero didn't fix it yet.
I've played through where the doom lord wouldn't spawn, then play though again with the exact same procedure and it spawned! So it's buggy/random at the moment.
To get it to (sometimes) work. Approach the plateau to trigger the first cut scene. Walk back to the plateau to trigger the portal appearing. Wait 1 hour and you should get a message about the portal opening (nothing actually comes out by itself unless you go near it so don't worry). Walk up to the portal and the demon lord appears!