I'm just going to leave Aldo's backstory mostly undefined here and instead highlight a few things. Astrology allowed him to make order out of chaos by way of predicting outcomes - when so many other things in his life were unpredictable. When The Darkening took place he began to notice that the stars had begun to change as well. This caused him much distress as the art he once depended on was rendered entirely useless overnight.
He wanted to know why so he came to the conclusion he'd need to reach Ymph, said to be where the Darkening began, and where he might find answers. He joined the Society of the Ordered Mind 's expedition to the surface. Though it was ultimately unsuccessful it would define him as an ally to the Ordered Mind. He would later make an attempt to join, though many of the Society didn't appreciate his radical ideas. His hopes of violently force the myconid colonies to relocate themselves near Traensyr, thinking they would lead to famine; were met with dismissal and hostility.
A mixture of this rejection and me being unwilling to wait two plus weeks for an app to pass or be declined led Aldo to come to the conclusion that the Society wasn't willing to do what it took to get him to Ymph. He joined the Watchers where he hoped he could find more traction for his expedition and get some revenge on the Society who spurned him. There he worked to turn Sanctuary into the perfect incubator for his expedition.
- By promoting the Black Guild and the idea of the House of Knowledge expeditions he could carry out the expedition in an official capacity
- By persecuting mercenaries and unaffiliated adventurers he could help swell the ranks of the Watchers and Warders who'd join him on his expedition
- By being an onerous fine collector he could swell city coffers for funding
- By taking some very shifty tips on the side he could fund his own supplies and equipment.
- By capturing and interrogating Stewards and savages, he might learn the secrets of their former home that would prove useful.
- By forcing the myconid lair to move near the Lowerdark, he hoped to cause a famine on thrall plantations and ease the passing of his expedition.
Aldo's search for answers came to a conclusion sooner than he expected. During an investigation into a disappearance, Aldo came across very bizarre and powerful magicks. An enchanted painting of the stars was found and in his attempts to steal this painting away, he was sucked inside and thrown into the void of space.
(http://i.imgur.com/4xICtcg.jpg)