This happens just during the world change that is caused by you the player! If the world change is showing, "The war is in a stalemate and no faction has any advantage" You need to make sure the difference is greater than 400 for one particular demon (or one side) so then their lords princes can spawn next server save.
Beware, it can take some time for you to kill all the princes. The spawn is pretty rough, only 3 princes spawn on the top floor but you can sit on the bottom while you wait for the respawn which takes typically 4-5 minutes. So typically it will take 3-4 hours to completely just 100 princes. I did some research on this as I don't have any personal experience but I plan to kill the princes soon myself:
You can ask Guide NPCs about the state of the demon war in order to check its current state.
The Demon Wars can be in 5 different states:
In order to change the state of this world change, there must be a difference in kills between the two demon factions at the
Server Save. The difference needs to be at least 100 to progress to the next stage.
If the war is on a stalemate and the difference is greater than 400, however, the Demon War will go directly to the stage where the Princes spawn.
To find where the spawn is:
Go to the Hero Cave, north of the city of Edron, here. Follow the common path to the main floor where many of the Hero Cave Quests are found. Go through the portal here to get into a safe room with the NPC A Dark Priestess. In this room there are stairs which will lead you to the middle of the Demon Wars cave.
To the east the Askarak Demons reside, and to the west the Shaburak Demons.
Sources:
Copy and pasted material from: https://tibia.fandom.com/wiki/Demon_Wars_World_Change
Great tutorial on the whole world change, please watch: https://www.youtube.com/watch?v=xS4nt4_WPEI