I said in order of difficulty, but it should be randomized with a % chance based on the level of difficulty.
So say, there is a relativly high chance (40%) it will spawn a group of Time Wights and Petrified Zombies, then a lower chance (30%) it wil spawn a few Kobold Commandos or Entropic Death Knights, then 20% to spawn 1 or 2 "almost boss" monsters (Ayssal Fire Mephling, Spellstiched Alhoon, Baelnorn Demilich), then a 10% chance to spawn a boss.
The percentages could be based on the number of PCs present so that if there are only one or two PCs in the area the chance to spawn the lowest level creatures rises to 50% and the chance to spawn a boss lowers to 5% for instance. If you got 8 PCs in the area the chance to spawn a boss could raise to 20%.
Also, the spawner could keep track of what it spawns so if it happens to be spawning the lowest level creatures over and over the chance fo it to spawn high level ones should increase.
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