Between Chronicles and Legends, what made Raistlin aspire to godhood?

In the concluding volume of this trilogy, the reader finds out what, exactly, Raistlin did after he abandoned the Companions to their deaths in the War of the Lance. Dragons of Spring Dawning shows Raistlin concluding a deal with the mysterious Fistandantilus, and the Legends trilogy shows what he did with that alliance, but until now there has been no explanation of what came between the War of the Lance and the Dwarfgate Wars. What power made Raistlin the Master of the Past and Present and allowed him into the Tower of High Sorcery - and a place in history as he pursued a place in the cosmos as a god.