How to let players feel the "power of belief"?

Question

In Planescape, more than anywhere else, thought has the power to reshape existence itself. That is an incredibly attractive notion and one that made me want to start an adventure there.

My players don't know much about Planescape or Sigil (other that you can find a gate to everywhere else) so I set-up a very simple hook that transported them unwillingly from the Forgotten Realms. Now they're in a strange place and forced to learn stuff the hard way.

The thing is, I'm not sure I know all that much about Planescape myself. Sure, I read the dnd-3.5 campaign setting from Planewalker and found it fascinating, but I'm not sure how to convey many philosophical aspects of the setting. Mainly, the power of belief.

From what I read, a single person with a unique belief will rarely ever notice the effects of his own thoughts. Since my players are figuring it all out as they go along:

How can I show them the power of belief?

So far neither the characters nor the players (I think) know that belief can shape reality. I'd like to find a way for them to figure it out.

Answer 1

It's called Harbinger House

One of the Planescape modules, Harbinger House is an exploration of the Believers of the Source (Godsmen), who believe that every person has within them the potential to become a god. The titular house is a sort of asylum for the "nearly there," and the adventure centers around a plan to exploit the potential in the House for the cause of evil. At the adventure's conclusion, a new god can be born right in front of the party - a testament to the power of both faith and philosophy to create change and bring something new to the infinite Planes.

If that's insufficient, or you'd like to try something else...

Show them something big

If you're willing to get creative, you can turn the dial back on your setting and let the PCs visit the Harmonium in Nemausus, third layer of Arcadia, shortly before the faction's militant activities push things to the brink and topple the whole layer into Mechanus. It's a great opportunity to explore creed vs. belief and come away with an understanding that merely wanting something and saying so does not carry the power to effect change.

...or something small

Another module, The Deva Spark, explores a radical transformation as a result of unlikely circumstances. The conclusion of the adventure is a minor thing in the grand scheme of the cosmos, but it is the beliefs of the party (in the form of the action they choose and the reasoning behind it) that determine the outcome.

Then, let them try it out

Set up an opportunity for them to start a small, unusual or harmless rumor in Sigil, perhaps inadvertently. Give them a few opportunities to run into it down the line - perhaps distorted or made more elaborate, in the manner of the telephone game. Let their interactions determine if it stops in its tracks, reverses, mutates or grows stronger. At some point, have them come face to face with the bizarre results of the rumor they started and may have unwittingly or deliberately fueled - they might even end up encountering bizarre alternate versions of themselves into the bargain!

Belief is not a short-term thing

Telling a kid the moon is made of green cheese isn't going to make it happen. Most of the time it's subtle, which makes the bigger moments all the more powerful. The power of belief isn't whimsy - it's a paradigm shift, on scales ranging from personal to cosmic, from accidentally renaming someone to causing whole chunks of planes to shift their position in the Great Wheel. Use the power of the slow-play - make the tension of a situation close to the criticality point into a plot element and have fun showing how and why things have been pushed to the brink. Look at a gate-town about to topple into its corresponding plane, or a tiefling warrior at risk of becoming a full demon and the sorcerous alchemist who has trapped her in a jar in order to attempt to extract demonfear from the process.

Answer 2

Give the players Belief Points.

Every DM worth his salt knows that showing > telling. But what's better than showing the players something? Let them DO something. A player character that's roleplayed at least half decently will belief in something. The right to freedom, the absoluteness of the law, that lying is wrong, giving money to the poor, that sort of thing. Having your players live up to this can make for a lot of fun roleplaying, but it not always comes naturally. But if they know that belief can reshape the planes... well, you'd be surprised at how eager they become to stick to their beliefs. Hence, Belief Points.

Taken from the Planeswalker's Handbook (Planescape book, page 142-147), Belief Points are given to the players if they stick to their beliefs, but at a cost. They are handed out at the DM's discretion and should not be granted frequently. For example, someone might believe that you must always answer questions truthfully, but that does not mean that if they answer a question truthfully that they would gain nothing from if lied about, you do not get a Belief Point. But if this were to come at great cost, personal or otherwise, then you do get them.

For example, if a Fiend were to ask "Am I a fiend?" and the character answers this correctly, this does not grant Belief Points. But if they were to be asked something that would result in the character's friends or allies being hurt based on the answer this would grant belief points. You can award one point for adhering to a belief but at not too great a cost, two for cases when there is a significant cost, and three in massive costs, perhaps even at the risk of the character's life.

But what do Belief Points do? By spending one a character can automatically succeed on any die roll (attack roll, saving throw, skill check or whatever you allow). This indicates that things go better for those with conviction. If a player spends three points at the same time however they can ask the DM a simple question regarding where the player is, the hazards they might face, what the enemy might do next and so on. This symbolizes a gut feeling, or a leap of intuition. Of course, the DM is not required to answer or can alter the cost to reflect the importance of the information.

Do make sure that the players can handle this. If you have conflicting beliefs in a party players will end up at odds with one another when trying to stick to their beliefs, and that's no fun for anyone involved. Furthermore, acting on a belief should not undermine the party or the progression of the plot. Also, players should not be forced to stick to their beliefs, but not doing so just does not grant Belief Points.