Threshold crest

s are cerulean blue stones that allow a city to plane shift.

Properties
The "threshold crests" are magical artifacts which allowed the city of Syngorn to pass from the Prime Material Plane into the Feywild and back again.

The Campaign of Vox Machina
Threshold crests were first mentioned when Vox Machina entered the city of Syngorn while it was within the Feywild. As they approached the sealed city gates in the outer wall, they noticed they each bore a massive emblem of a crescent moon flanked by two trees over a deep cerulean stone about two feet across. The twins identified them as the threshold crests, the enchanted crests that enabled the city to travel back and forth between the Feywild and the Prime Material Plane. Artagan (as Garmelie the satyr) tasked Vox Machina to procure him a piece of a threshold quest as the price for his aid in escorting the party to the Shademurk Bog. The party skirted this requirement by giving Artagan a piece of the threshold of their room from which Keyleth had used Stone Shape to cut out a crest.

The party noticed another threshold crest within the city of Thar Amphala. It was a deep cerulean blue gemstone, about two feet across, embedded within the wall, and was very similar in size, installation, and functional placement as the threshold crests adorning the exterior of Syngorn.

Campaign Two: The Mighty Nein
While pursuing the Tombtakers across the island of Foren in Eiselcross, the Mighty Nein encountered an extremely large threshold crest within the Aeorian ruins referred to as "A5". The chamber it was found in was lit by bright blue light from the foot-wide gem in the center of the ceiling thirty feet above. Caleb detected a strong aspect of teleportation magic in the room, and the gem had a very strong aura of conjuration. He was able to identify that the crystal was part of the largest threshold crest he'd ever seen, and that it would take multiple crests to plane shift a city. The Tombtakers chiseled out the crystal and took it with them when they left.