The story is that he escaped from the drow. As a young svirfneblin, he was apprenticed to an alchemist and learned the basics, but before he reached his majority, drow conquered his burrow. he and many of his people were taken as slaves. The drow either didn't know or didn't care about his alchemical skills, and set him to serving in the fields where the mushrooms grow. So long as he met his quota (which took most of his time), they were left largely alone (although they were watched). He was able to cobble together some alchemical substances by using cavern pools, mushrooms, bits of ooze and various algae; but it still wasn't enough. He learned what he could from other prisoners, picking up tips on survival in the darklands, then he faked a disease so that he would get sent to the Brimstone mines (where they send those slaves that are near death anyway as it is extremely dangerous there). The guards do not enter the mines; they count them in and count them out and any differences is assumed to be due to death from the poisonous fumes or the oozes. But brimstone is an important alchemical substance from which many extracts are made; with enough supplies he was able to make his escape.
Alongside the mines lay a fast-flowing river. Despairing slaves were known to occasionally throw themselves in to end their painful lives. using a Touch of the sea extract, he was able to navigate the river a good long way, avoiding many of the rocks with his superior swim skill. As an added precaution, he quaffed a no scent potion before leaving the river.
and it worked. He was free, but alone in the wilderness. He does not know how long he wandered, but each time (afraid of pursuit), he chose narrow paths, paths that the drow could not get through. He also chose paths that went up, reasoning that they would expect him to follow streams and rivers down.
After many months, maybe even years, he came across a Duergar patrol.
