How to plan the CR of an encounter your PCs can't win?

Question

I'm going to create a combat encounter that my PCs are very unlikely to win, with the ultimate result of their "dying" in combat leading to their capture by a yet-to-be-revealed villain. I don't want them to be completely unable to win, as there is some fun story I've thought up that they can see if they do beat the boss, but ultimately there will be no penalty for them losing the encounter (besides shame) as they will ultimately escape their captor and regain any lost items, wealth, equipment, etc.

I'm tempted to just use the CR for the enemy they'll be facing, and give them XP accordingly, but given that they could just phone in the fight (they should have a pretty good idea that they're going to lose, with some heavy hints that someone is trying to capture them) I don't know how to rate it and reward them.

If they do give the fight their all, should I treat it as normal, and only reduce CR/XP if they surrender/don't try? Is there a better way to put them in an insurmountable position without them giving up?

If it matters, I'm a new GM with mostly new players, playing a Pathfinder campaign.

Answer 1

Your question asks how to "determine CR", but it sounds like your actual goal is to figure out how much XP to award.

Rules As Written, you only award experience points for "defeating monsters, overcoming challenges, and completing adventures". Unless the players defeat some of the enemies involved in the challenge, Rules As Written there's no XP reward.

However, you have the ability to give your players Story Awards:

Feel free to award Story Awards when players conclude a major storyline or make an important accomplishment. These awards should be worth double the amount of experience points for a CR equal to the APL. Particularly long or difficult story arcs might award even more, at your discretion as GM.

If you decide to not do the Story Award for the battle itself (because you're worried the players might not try very hard), your other option is to add some sort of achievable goal to the battle. The players can't avoid capture, but they can wreck the villain's favorite magic item, or they can thwart the villain's plot, or they can kill some of the villain's henchmen. (It's a good idea to add a goal like this anyway, so that the players don't feel too frustrated by having to fight a losing battle.)

If it's too difficult to cue the players in to what is expected of them, consider simply pausing the action and explaining what's going on out-of-character: "this is a difficult fight and you're probably going to all get captured. But if you can find a way to thwart the villain, you can still get experience points for that, and it could help your group out in the future."

Answer 2

From Gamemastering,

Difficulty: Challenge Rating
Easy: APL –1
Average: APL
Challenging: APL +1
Hard: APL +2
Epic: APL +3

In order make an encounter they're "very unlikely to win" you'd want to start at +3 and possibly go to +4 or +5.

What's in a CR?

Taking the big bad guy (or their henchman), calculate their CR either by their Monster entry or by their gear loadout plus Class levels minus 1. Now, fill in the other CR by converting to potential experience. A truly dangerous encounter will involve a control mage and a bunch of minions. Being able to disable PC's is the lifesaver of villains everywhere, and the best way to raise difficulty is to leverage action economy. Throwing twice as many creatures at 1 or 2 lower CR is a much more difficult encounter (reflected by the CR Equivalencies table)

Finally, increase the CR by also putting the PC's in a disadvantaged location/situation. This is one factor that could easily be mitigated by the PC's. Good ideas include "the ranged enemy is at the top of this switchback trail" and "the necromancer cast Unhallow on the area".

The PC's were captured, what now?

Strictly speaking, the DMG suggests that PC's gain experience from "...defeating monsters, overcoming challenges, and completing adventures." However, as @MikeQ indicated, Paizo (and the 3pp who make Adventure Paths) typically award experience any time the PC's survive an encounter. This is prolific in Traps; even if you don't spot it and instead get hit by it, you walked away from the encounter and now know more than you did before you survived a spring loaded axe to the face. Why cause issues, to you as the GM, with planning your next couple adventures' difficulties around if they succeed or not? Give them the experience regardless. It will make your life easier and help the players bandage their hurt feelings when they realize their PC's aren't lost, and in fact are more ready to counter this particular threat when it comes to that.

One more thing

It is surprisingly difficult to knock players out on purpose at levels higher than around 5. Damage quickly escalates over the negative hit point threshold. Make sure to include at least one way of 'saving' a downed PC (such as Breath of Life) and that your minions deal at least some nonlethal damage to increase that knocked out buffer between fighting and death. Consider a focused grappler to take down a pesky character or two with Grapple>Pin>Tie up. It's not fun to be tied up, but losing in general won't be a good experience. This also provides another opportunity for PC's to prepare properly and reduce the difficulty of the fight (in this case, by preparing Freedom of Movement, Liberating Command, and/or Litany of Escape).

Answer 3

A standard team of 5 PCs at lvl X has a CR of X+3. This means the team should have 50% of chances to loose a fight against a CR X+3 encounter. If you go beyond that the enemy is supposed to be more likely to win. If the encounter is of CR X+7 then it can be seen as an easy encounter for the enemy, corresponding more or less to "no practical chance for the PCs to win".

However this is only the theory. In reality a lvl 1 group has no chance at all if you make them fight a CR 4 solo monster and a lvl 20 group will laugh at a group of 12000 goblins (which is theoretically CR 25). The only way to know how hard it will be is to test it, learn how monsters work, play a lot, and with experience you will finally be able to tell how strong each encounter will be. From my experience the general rules I got are that usually:

• Encounters that involve only one brute (like a bear, a troll...) are harder than their theoretical CR.

• Encounters that involve only one mage/specialist (anything that relies mostly on its magical abilities or anything that is not direct attack) are very random. A high CR one can go without a scratch and a low CR one can result in a total party kill.

• Brute monsters of individual CR lesser than the level of the PCs minus 4 are not significant in the calculation of the total CR. Mages/specialists are significant.

• Confusion effects are more powerful than what seems to be taken into account in the CR calculation

• If the main feature of a monster is useless against the team, then it can go from CR-1 to not a threat at all, depending on what are its other options.

Answer 4

Capturing the PCs

If the goal of the encounter is the capture the PCs, you should include some powerful monsters - ones they won't be able to take out easily. But these monsters should have something like paralyzing venom, sleeping spells, or nets to restrain the PCs.

If it's a fight they're going to lose, I wouldn't worry too much about XP. Give them the XP for any enemies they defeated.

Above all else, make sure that through all of this, everyone is having fun. Let the players look cool, even as they're losing.