The Festival of Masks is a holiday in the Temphere campaign universe, temporally adjacent to the real-world Halloween. The festival celebrates the defeat of the prince of the Shadowfell by the trickster god Dalone, and explains why crossovers with the Shadowfell are exceptionally rare despite Temphere's otherwise frequent bleed with other planes.
The Legend
According to the backstory of the festival, a very long time ago the dark realm of the Shadowfell frequently crossed over with Temphere. Demons leaked into Temphere through these crossovers, and while not all of them were hostile, most were, and caused tremendous chaos in the material world. Eventually the god of trickery, Dalone, decided to do something about the problem. Donning a mask to hide his face, and illuminating his path with a candle inside of a pumpkin, Dalone traveled into the Shadowfell to speak to the demon prince who ruled it.
Dalone struck a bargain with the prince- the two of them would play a game, and if the prince won, the god would serve him. If the god won, the prince would seal his realm away, not allowing it to crossover anymore. The prince couldn't see the face of the one he was talking to, and only knew that it was a god- not the god of trickery. The idea of having a god under his control was too much to resist, so the prince agreed. The exact game the two play varies from telling to telling, with some versions of the legend even asserting that Dalone deliberately didn't reveal the nature of the game, so that the prince could be depicted being humiliated in as many ways possible. Regardless, Dalone won the challenge, and in a rage the prince tried to attack him. Dalone threw his pumpkin to the ground, vanishing in the flash of magic that followed.
The Festival
According to tradition, everyone attending the festival of masks will wear some sort of mask. A simple jester eye mask is perfectly sufficient, but the masks can be and frequently are extremely elaborate, with some individuals even wearing elaborate full body costumes. Pumpkin carving is also a frequent part of the celebration, and all manner of games and carnival booths are set up around the town or village. Candy and prizes are usually available to those who do well at the games. The area is decorated in fall colors, with various spooky themes.
Different locations will also observe individual traditions as well. In Skyhorn the carved pumpkins accumulated during the day are set into small boats and floated out into the ocean at the end of the night. In Caellond an object is stolen from each festival-goer at the beginning of the festivities, and they must collect tickets from the various games to win the chance to visit the "prince of the shadowfell"- Valanthe Liadon in disguise- to get back their possessions.