In the past two sessions, the orcs have sustained some heavy casualties:
In Into the Orc Hole II, the PCs killed about a dozen orcs, mostly big "brute" orcs, along with four goblins, two devil wolves, and two half-ogres.
In Into the Orc Hole III, the PCs killed about 50 orcs, most regular orc warriors but one orc sergeant and one spellcaster.
Given that Galen's estimate is that maybe 300 orcs warriors are up in the valley north of Felltower, this is about 15-20% casualties. All dead, only a handful wounded and escaped. Unachimba asked how many forces/tribes could survive suddenly losing 10% of their number?
It's a good question.
Is that it for the orcs?
Losing 15% of your numbers in a couple of weeks is quite a serious amount of damage. But will that stop the orcs? The real question is, are casualties the orc's center of gravity? If so, then it's likely the orcs will back off, retreat from the dungeon entirely, or attempt some non-conflict way to resolve their goals.
But what if they aren't, ala - just to choose some easy examples - the Soviets in WWII, the Chinese in the Korean War, or the Romans during the Punic Wars? What if losses and casualties are just seen as a combination of the price of doing business, the cost of poor tactics, or a renewable resource? What if the orcs see combat casualties the way the PCs see money - something you spend in battle (via potions, paut, consumables) and then forget about?
Then the orcs aren't done. Even with a lot of casualties, if they don't see casualties as a big deal, it's not a big deal. They reproduce like mad and they have a totally callous attitude about death (theirs and others).
The PCs really haven't figured out what the orcs want. During their friendly phase the orcs weren't very good about explaining what they wanted. It's clear the grunt-level orcs don't really know the details beyond a fuzzy level, and those who know aren't talking. And aren't interesting in having the PCs accomplish it for them.
Ultimately, casualties can determine the outcome of the PCs vs. the orcs - the PCs can inflict so many casualties that it has to matter, even if the orcs aren't concerned by any individual or small group loss. But unless casualties in and of themselves are something the orcs are concerned about, it's probably not enough.
A big question remains - do the PCs feel satisfied with having given the orcs a good hard drubbing, or do they push the issue, risking casualties and poor loot (the orcs are low on swords, and high on low-grade-metal mail and axes) in order to push the orcs past recovery?
We'll see next session.