Their strongest and most powerful spell was simply called "P", and casting it meant instant death for all foes. Little is known about the inner workings of the Cult of Runes. From a letter written by the Druid Selzhanik in the Book of Amber Runes, it is hinted that the cult has a hierarchy of nine circles, and each circle has its own set of runes. The fifth circle has jade runes, the sixth amber runes.
It is unknown what happened to them afterwards. They were probably absorbed into the bigger mage community of Britannia. Some of them could also have been among the settlers heading to the Serpent Isle.