I usually keep it vaguely defined, but I do have one race I went into pretty thorough detail.
The Yoon-Suin campaign setting has many strengths, and one of them is that it leaves the individual DM a lot of room to create their own version of Yoon-suin. The origin of slugmen wasn't explained, but it became an important part of the lore of my game.
the detailed version is at the link below, but tl'dr, the slugmen were created by a archmage as servants and assistants. This archmage and his allies were in conflict with krakens, keeping them from going upriver to find the "divine source". The wizard eventually passed away, but entrusted his slugmen servants with the anti-kraken mission. This was thousands of years ago.
But unlike say, the owlbear, the slugmen
remain an artificial race. They don't breed true. Untreated, their eggs hatch into mere slugs. But the Wizard taught the slugmen how to alchemically treat their eggs so they hatch into slugmen. An important part of their trade policy is obtaining those ingredients
and obfuscating
why they need those ingredients.
Here are my notes on slugmen biology and reproduction, and how it affects their culture tl:dr: slugs are not mammals, and slugmen are not a...
slugsandsilver.blogspot.com