Words and Oaths

On The Shattered Isles, words can hold power. Invoking ancient powers before speaking will cause the universe itself to take note of you and listen to what you have to say. This is commonly used as a way to make solemn promises and pledges. A promise witnessed by the universe holds power, and breaking such a promise can have serious consequences. These can include negative levels, permanent loss of characteristic or skill points, lowered max HP and spell slots, or being cursed.
Any player or NPC can invoke these ancient powers at any time, and there are many ways to do so. One example is the rule of three promises. Making a promise three times in quick succession starts the process, and the recipient binds the promise by stating "Thrice heard and witnessed." Many fairy tales tell of tricksters fooling powerful people into making such promises.
Another example is simply stating that you are invoking the ancient powers. You can call upon the stars, the moons, or the earth itself to witness your solemn oath.
The most important aspect of this mechanic is simply making it clear to the GM what your intentions are, and to word your promises carefully. The universe is very literal and cares not for context or expression.