Incursions
Whether it was planned or it was a side effect of the Sundering, after that event, transitioning to other places, such as the realms of the Fae, the Shadowed Land and the various heavens and hells become more difficult or nearly impossible without extremely potent magic. At least, most of that time, that is true.
But sometimes, the walls between the world and one (or more) of the worlds beyond thins. One of the reasons that the study of astrology is such a refined art in the Sea of Stars, for the alignment of the stars predicts and reveals these periods of thinning. Yes, the stars will be right.
Even with the walls between worlds being thin, it usually takes some sort of event to create a door. A storm, a stone circle, a good old fashioned ritual, these and more can all act as the final key to open the door between worlds. These doors can be opened by accident as well as by design and occasionally dangerous -or just odd- creatures are released into the world. Sometimes they are snapped back to their side when the alignments change, other times they are trapped.
These alignments are the best time to summon creatures to be bound. Summoners call and bind angels, demons and other more exotic metaphysical creatures for different tasks, usually considering them to be nothing more than tools for various jobs. It is said that the Empress disapproves of such dealings with outer beings but she has not imposed legal sanctions against such actions.
Notes: In one of the earlier adventures, the player characters had to deal with a guardian angel who tried to warn them off and almost killed their best fighter.
Photo by Forest Wander and used under a Creative Commons Attribution-ShareAlike 2.0 Generic license
Sea of Stars RPG Design Journal 