What is the center of gravity of a group of NPCs?
In other words, what's the thing they center on, which, if lost, will effectively destroy the cohesion and will of the group?
The NPCs in my case are the factions and grouping in the megadungeon (and amongst outside intruders).
The basic idea is to decide what will cause a group to cease functioning other than total extermination. Or what will keep them coming back no matter how many of them you kill off.
Here are some examples of what I mean:
Let's say I have a mish-mash group of NPCs led by an evil cleric seeking a powerful item in the dungeon. The evil cleric might be the center of gravity. If he's killed, the group will disperse. It can survive anything but the loss of the cleric. This is traditional kill-the-head-and-the-body-will-die stuff.
On the other hand a group of hobgoblins might have a center of gravity of their females and young. As long as they're okay, they'll keep coming back. This is a social center of gravity.
A group of trolls might have a specific cave system they need to hold, because that's where they get their water. If they get cut off from that, they'll leave the dungeon (or die off, or whatever). This is a logistical center.
A rival party of adventurers might have a goal of seizing a certain amount of money. If they can get it - even if through bribes, scavenging treasure left behind by the PCs, etc. - they'll head back out. Their center of gravity is money. They might also have a key member or two, that, if slain, will cause the others to leave.
A faction seeking something in the depths of the dungeon might have an external center of gravity - they'll keep going until that item is recovered, no matter how many minions they need to deploy to do so. Unless the campaign either encompasses their origin point, and thus their true center of gravity, or the PCs eliminate the goal item, they'll keep popping up.
Mindless undead, animals, scavenger types - they might have none. Kill off 9 out of 10 of them and the 10th one keeps on keeping on. They have no center of gravity beyond just being.
It's something worth thinking about for NPC groups, so they won't just be hordes you need to exterminate to get rid of.
(This was occasioned by me reading a military biography of Hannibal. Great example - he continuously attacked Roman armies in the field, and trashed them. He'd pry away some of their allies, too. But since the strategic center of gravity wasn't the Roman Army, he couldn't win because they'd lose but wouldn't surrender.)