Quote Originally Posted by Rhynn View Post
1. Never, ever think that you know or can control the outcome. Don't force a defeat, that's stupid, unfair, and counter to the point of playing a game.

2. Intelligent opponents with an interest in taking PCs alive, as prisoners, for ransom, as slaves, etc.

3. Conflict within social constraints: cattle-raiding, feuding, dueling, etc., where deaths happen but killing a surrendered or defeated opponent is considered wrong or even murder.

4. Be aware that your players may not even imagine that non-fatal defeat is possible. You need to introduce the concepts (ransom, wergild, etc.) early and explicitly, and then positively reinforce their use. Don't suddenly have opponents murder surrendered or defeated PCs, because that will immediately train the players to never surrender.
As is so often the case, I agree with Rhynn. (And with Sir Pippin, but stuffing two quotes in this post felt like overkill. Sorry, PB!)

You can't force the players to surrender or give up without a fight. Unless they know that a fight is suicide, they'll probably go into it given half an opportunity. Warning them in advance may give the wrong message, and make them convinced that the warning was a plot hook, not a deterrent. Killing one of them to show how powerful the enemy is is more likely to make the rest angry, not cow them. As Sir PB notes, players are often simply too aggressive to be sensible.

As Rhynn suggests, really your best option is to use a fight in which a loss doesn't mean death, such as a structured duel. This will allow you to use a much stronger opponent without the lethal outcome. It will impart onto the PCs a knowledge that they are not the unstoppable force they think they are. Maybe it will even humble them.

But remember, they could always defy the odds. Even if there's only a 1/20 change of victory, if the PCs score that natural 20, they are entitled to that victory. Be prepared to give it to them, even though it ruins your plans.

As an aside, I think it's bad form to "want[] players to experience defeat[.]" If they've got power and tactics and continue to rock on, as long as you can keep producing a reasonable challenge, it's not your job to teach them humility. As long as they have fun, and you have fun, why mess with a good dynamic? This isn't some sort of adversarial game where you earn points by brutalizing the players. (If you want that, go play Kobolds Ate My Baby! Also, play it anyway, because it's awesome.)