Imagine a pool some ten feet wide and three feet deep, filled with holy water, in the center of a chamber with crystal walls and dome ceiling. The chamber (and whatever building it’s part of) is constructed so as to refract all outside moonlight into the pool, regardless of the moon’s position in the sky. Sunlight does not pass through the materials of the dome.
The light reflecting from the pool illuminates the whole chamber, creating a unique cycle of dark and light: a night/day cycle based on the rising and setting of the moon, and ranging in brightness as the moon waxes and wanes.
The pool’s purpose is to focus not only the moon’s actual light, but its mystical qualities (dependant on gameworld) as well. The water becomes a sort of “liquid light,” used to power spells or communicate with the lunar deity.
Lunar Zenith Event
Once per lunar year, the moon shines directly into the pool through an aperture in the dome. An intense beam of moonlight, the full width of the pool, is reflected straight back.
This is the most concentrated energy source of the year, and it is the center of ceremonies and powerful spells. Depending on the theology of the gameworld, it may also be seen as a “transmission beam” – point of contact with the lunar deity – an earthly response or
- As an information-site. The moon is a perfect symbol of mystical knowledge; it illuminates, yet leaves the night-time mysterious. A place where the moonlight is quasi-physical, your PCs could gain any piece of obscure wisdom, whether this information comes from direct interaction with the altered water in the pool, or from a class of mystics who tend it (in which case the pool becomes more of a setpiece for an important NPC encounter).
- As a Quest Item. For some reaso your PCs need special intervention. One of them needs healing from a magical injury. Their wizard needs to bathe in the waters to level up his magic. Or the party’s been told they need to be touched by your gameworld’s lunar deity to complete their quest. In all of these situations, getting to the pool is an objective in itself. In this case, the pool should probably be in an exotic and remote location.
- As someone else’s plot objective. Last week in my pirates campaign, the moon was a maguffin the villainess wanted to control. Because the players were following her, and didn’t know anything abou the pool until they got there, we got to do a bit of worldbuilding together – both indirectly, when they started asking questions about it in game, and directly, when I gave the PC who was from there carte blanche to say what she thought the villainess might try to do with it.