Hi,
Years ago, I made rules to convert Avernum to MERP/Rolemaster 2nd edition and later also RMFRP. The most problematic thing about that was the magic system: Rolemaster magic system doesn't feel like it fits for Avernum. In Avernum just about everybody can some learn magic with ease and there are no spell fumbles in the games.
I can check later if I can find my notes.
I agree with Vydaera's list though. Here's a short list of features to be considered:
1. Campaign time
Consider that as the time goes onward, more and more humans are born in Avernum and have lived there for their entirely lives
Avernum 1
- All races are in war with each other in Avernum.
Avernum 2
- Some Nephilim and Slitherikai have allied with Humans allowing mixed race parties
- Vahnatai reveal themselves, not very suitable for PCs though due to their demeanor at this time
Avernum 3
- Outside world possible to be explored by Avernites
I haven't played Avernum 4-6 though I read their backstories, I have no specific insight on those
2. Race
Humans
- No special features
- Originating from the surface: skin increasingly pale with the amount of time spent in Avernum
Nephilim
- Nimble and fast, good with bows
- Originating from the surface
Nepharim
- Sub-race of Nephilim
- Stronger than Nephilim but infertile
Slitherikai
- Big and strong
- Their physique allows them to wield large weapons with ease
- Fire-resistant
- Indigenous to the caves
Vahnatai
- Smart, able to craft magical crystals and extremely sharp weapons
- Attuned to living deep underground
- (Not very suitable for PC races due to being ancient and alien)
Some monster races of Avernum:
Goblins
- Small, weak, stupid, typically hostile
Intelligent spiders
- Friendly ones: Silly, simple, quirky, typically no magical powers
- Aranea: Evil, magic-wielding
Gremlins
- Like insane fey-folk with razor-sharp talons
Ogres and giants
- Big, strong and usually stupid
- (But I can't remember, are there giant mages?)
3. Culture/Origin for humans
Surface dweller
- You were a human who was, rightly or wrongly, exiled to Avernum
- Possibly longing back to surface
- Need background options on what they were doing before and why they were banished
Avernum-born
- Possibly naturally attuned to living in caves
- Who were your parents? Were they exiled to Avernum? (do you know) Why?
4. Class
I'm unfamiliar with Pathfinder's class system. I've only dabbled with DnD 5th Ed. Of DnD5 classes, I would omit Monk because in my opinion unarmed combat does not fit the setting of Avernum.