Saradon is correct - slann were made by the old ones to oversee there plans on this world and to control the infinite spill of magic
There was no chaos original just giant gates at the north and south pole which held chaos at bay. Until the old ones vanished and the north gate collapsed. Freeing chaos into the world
The demons are more powerful than the slann, but due to the vortex created by Caledor the Mage drained a lot of the magic off the world reducing the world's power
The slann can also tap into something called the great reservoir . Which is a gigantic store of magical power stored by the Lizardmen within there temple cities, plus they are extremely long lived using most of their time in mediation. There are three generations of slann with the first being the most powerful but mostly gone