Threshold crest

Threshold crests are cerulean blue stones that allow a city to Plane Shift.

Properties
Threshold crests are magical artifacts that operate in conjunction with each other to transport everything within their triangulated area across the planes. They do not open portals or doorways that people or objects can pass through, but must be ritualistically bound to a location and then can transport it.

Vox Machina Origins Series III
The dragon Skysunder traveled from Westruun to the Frostfell with a shard of a threshold crest she wore on her forehead. She used a pointed end of the shard to cut a portal between the planes, which remained open for only a few moments. Vox Machina killed her in the Frostfell, and Vex'ahlia used it to return them to Exandria. Eskil Ryndarien identified the shard as a piece of threshold crest and noted that it was drained of its magic.

Campaign 1: Vox Machina
In their pursuit of Riskel Daxio, Vox Machina entered a Clasp hideout and bargained for passage and access to Daxio with Spireling Warren using the threshold crest shard taken from Skysunder. Scanlan claimed that it would grant the Clasp a Wish, and while they saw through the deception, they also could tell that the shard still had magical potential and accepted the trade.

Threshold crests were first mentioned by name 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 the threshold crests, which allow the city to travel between the Feywild and the Material Plane. Syngorn's crests are two feet across and deep cerulean in color, and each sits under a massive emblem of a crescent moon flanked by two trees. Artagan (disguised as Garmelie) tasked Vox Machina to procure him a piece of a threshold crest 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 2: 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 gem (roughly one-and-a-half to two feet wide) 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.

The Mighty Nein found another crest in the ruins of A2 and took it with them. They later lost it when the Tombtakers stole their Bag of Holding, but in turn recovered the one the Tombtakers had previously found. They attempted to Teleport the crest to the Rexxentrum Archive of the Cobalt Soul, though it didn't arrive there. With the help of Yussa Errenis, they determined that the crest was in a forest, most likely the Pearlbow Wilderness. Essek Thelyss explained to the Mighty Nein that if the Tombtakers intended to use crests to transport the Cognouza from the Astral Sea back to the Prime Material Plane, they would have to use some other means of taking it there first.