However, if you simply must have a system I'd suggest using the "Faction" system from the EverQuest d20EverQuest d20 game (the paragraphs about the Faction rules are found in both the Player's Handbook and the Game Master's Guide, explaining the mechanical aspects respectively giving DM advice for awarding Faction points and describing already existing groups in Norrath), published by Swords and Sorcery Studio in 2002 (or sometime around that). The base line is a new stat - appropriately called "Faction" - that is tracked separately for each group. This Faction ranking ranges from -10 to +6 and is adjusted by the player's actions.
Example Faction adjustments:
-6 | Murder/Treason/Sacrilege
-4 | Arson
-3 | Theft
+1 | Recovering stolen goods/prisoners
+2 | Eliminating enemies
The important thing is that only actions that the NPC group is aware of affect the players' Faction ranking. So if the players cover up their tracks very well or do something stupid when really nobody is watching, their Faction ranking remains unaffected.
Obviously, there are several problems with such a mechanic.
- Gaming the system. If you allow minor reputation gains ("donating to the church", etc) the characters can simply buy their way into benefits with their gold and even compensate for crimes against the current group. This may lead to back-and-forth changes of Faction ranking between doing something bad and then paying to get rid of the penalty. It's similar to the NWN alignment system (which I personally found utterly ridiculous to begin with, but that's another story).
One possible consequence is that as long as the characters have enough money to donate, the group may simply ignore even heavy infractions (like e.g. murdering one of the group's leaders). The only way to prevent such a situation was to ignore the Faction ranking system, which then begs the question why it was needed if you wouldn't stick to it.
- Complexity. Such a system quickly grows out of bounds if you try to model relationships between groups. Imagine 4 groups: A, B, C and D. A and B are allied, as are C and D. A and C are hostile, while B and D are relatively neutral towards each other. Now the PCs complete a major quest for A which gives the group a big advantage over C. So, theoretically, the characters' Faction rankings should change with each group:
- A: big Faction gain for completing the quest
- B: lesser Faction gain for helping their ally
- C: big Faction loss for helping their enemy
- D: lesser Faction loss for hindering their ally
If you have complex networks of groups (and possibly even powerful individuals like dragons that follow their own agenda) such a system quickly grows overly complex.
- MMO-ification. The Faction system used in the EverQuest d20 game is a - more or less - direct but simplified adaption from the system used in the EverQuest MMORPG. Simulating social relationships in an online game simply cannot match the degree of detail possible in a pnp rpg, but rather requires abstract calculations. Therefore, using such a system at the gaming table my introduce a break of gaming style not expected by the players ("What do you mean we have to grind standing with the city guard before they will offer the next quest?")