It's pretty easy to do, but make darkness a non-hostile spell.
Why? It affects NPCS in bad ways. It can screw their AI up when they try to attack something that they can't see while blinded in retaliation for casting the spell; and if it hits a friendly NPC it will make all of them hostile :/
The 2da required to be editted to add another type to put any AoE you want to modify this way, so that they still break invis and stuff, but don't mess with factions. This could either be darkness only (2 minutes work) or maybe other AoE's like clouds (25 minutes tops)
If you're going to be using this hostile (On purpose) on 'friendly npcs', chances are there will be a DM to control them appropriately in reaction to the darkness which they may not have time to do faction changes before it all gets fubar'd.
Otherwise, chances are you hit them by accident and then you can't hang out in populated areas because the NPCS will crush you.
Please do this.
Concur.
Do not want