Let's create a new faction. For this example, the NPCs on this faction will be KOS to all PCs except Erudites, whom they will con amiable. These NPCs will also attack other NPCs whom primary faction is Heretic. If anyone kills these NPCs, we will have them lose 10 points of faction with their faction (the new one we're creating) and gain 5 points with Heretic.
Create a new entry in faction_list. Since we want them to start out KOS to everyone except Erudites, set their base value to -1000. This is barely KOS, so if you want to force more faction work to get above KOS, set is that much lower.
See table below:
1100 -> ABOVE
750 -> 1099
500 -> 749
100 -> 400
0 -> 99
-100 -> -1
-500 -> -101
-750 -> -501
BELOW -> -751
Create the new entry in npc_faction. This will be an umbrella for NPC behavior entries. Remember, this is different from the entry in faction_list. faction_list only identifies a core faction. npc_faction identifies a subset of NPCs on a faction in faction_list that have a prescribed behavior. There can be many npc_faction entries providing different behavior for NPCs, even though all those NPCs have the same code faction_list assignment. The key fields in npc_faction are primary_faction (which you will assign to the new faction you created in step 1) and ingnore_primary_assist which you will set to 1 or 0 depending on whether you want NPCs under this umbrella to assist other NPCs on the same faction_list allegiance.
So we wanted our new NPCs to like Erudites. We do this by adding an entry to faction_list_mod. This applies to any and all NPCs that refer back to the new faction_list entry. So we add an entry, using the faction_list value we created in step 1, a mod_name ([rcd]#) and a value. We want Erudites to be amiable to start. So we need to get them from -1000 up to at least 101. Depending on where you want them in the amiable range, the mod would be something like 1101+. So the new entry would be (id, (step 1 id), 1101, r3). Erudites are race 3.
So we decided we want the NPCs under the new npc_faction umbrella to attack other NPCs whose primary faction_list faction is Heretic (143). To do this, we add an npc_faction_entries entry. For us, this would be (step 2 id, 143, 5, -1, 0). This indicates that all NPCs under the umbrella we created in step 2, will attack NPCs that are under any umbrella that has a primary_faction of 143. The -1 indicates attack. If we changed that to 1 it would mean assist. A 0 in that field would mean ignore. The faction_hit is set to +5 so that if a PC kills one of our mobs, their faction with Heretic will go up. We also need to add a line for our own faction, so that PCs killing our NPCs will lose faction for that. That entry would look like (step 2 id, step 1 id, 10, 1, 0).
As the npc_faction_entries documentation mentions, these table entries serve dual purpose and might be better served as separate tables.
Simply change the npc_faction_id field for your NPCs to the new id from the second step.
Since we want these NPCs to attack other NPCs, change the npc_types entry for any NPC assigned to this faction so npc_aggro is set to 1. This field must be set to enable these NPCs to agro other NPCs.