You want to know how the demons are made, mortal? Ha, don't make me laugh. You past by where all of us were made even me, but you didn't know that did you? Of course you didn't, why would you? If you don't mind me asking, have you been the one destroying my forces and sending them back here? I bet you loved walking past the pools didn't you? All of those demons crawling out and lusting for chaos, bloodlust, and an insatiable hunger.
Demons, scum, bastards. I hate those things! Gladriel, get my longship ready we're going to kill those bastards right at the source, The Tar Pools!
Demon birth has been around since the creation of the realms, and nobody knows when that happened. This "tradition" if you could call it that hasn't changed over the years, but has become more frequently since the Broken Branch Event. With more demons coming out of the Pools and wrecking havoc on the other realms.
Once a demons dies its soul and spirit gets transfered to the pools, once there it will incubate for a timespan (varies for each demon type) where they're spirit and tar will mix and make the body. Once the incubation process was done, the demon will claw its way out of the pool and go back to wrecking havoc.
Every demon partakes in the Pools, for that is how they are born. The main thing about these pools is that every demon, from the Demon King all the way down to the Imps, are made here.
The only demons that get a gathering are the Demon Lords and the Demon King. Every other demon, with exception for the Hunter, are ignored.