Hellgate

Hellgates, or portals, are gateways made of obsidian, that led to the Nether. They seem to be linked to the source of the Sands, as well as Israphel and his cult. They are dangerous, as they produce the Sand where they are placed, and are used by the Cult of Israphel to attempt to spread the Sands in the lands beyond the Wall. The first gates were created by an unknown civilzation.

In chronological order, they are

The Ancient Hellgate
This Hellgate was discovered by Prof. Grizwald, before the Sand War, as he was research the ancient civilzation that created them. He accidentally activated it, causing monsters and darkness to start pouring out of it. The activation of it also activated ancient giant signal fires, (which may have been later used for the Wall) and caused both a a giant meteor to strike the area, and other disasters. It was destroyed by Verigan Antioch and his band of adventurers.

the First Ruins Hellgate
Built by the Cult of Israphel that was based in the Crumbling Ruins outside Mistral city, inside said ruins, it wa destroyed by K. Peculier, possibly alongside other members of Mistral City's guards. Later discovered by the Yogscast

the Mine Hellgate
This Hellgate, built in the mines under Mistral City, was destroyed by K. Peculier, using explosives, after he walked through it to seal it shut. The existence of this portal was covered up, and the mine was closed as a result. Discovered many years later by Old Peculier, and the Yogscast

the Hellgate of the Cave of Terror
Built by the unsupecting Yogscast, this Gate was how Israphel returned to the world. Used by Israphel later as an escape route from his Castle, he also took the kidnaped Daisy Duke with him through it. Destroyed by the Yogscast at the beginning of season 3

the Second Ruins Hellgate
Built in the same chamber as the previous Ruins hellgate, it was destroyed by the Yogscast.

the BBQ bay hellgate
Found in a cave underneath BBQ Bay, and linked to Jock Fireblast's ship, it was presumably built by Jock. The Yogscast entered this portal, fleeing from monsters and a trap activated by Jock, it lead to a metal box located in the nether, much to our protagonists surprise. The box has signs above the portal, but there has not yet been a chance to get a clear look at them.

There may be other hellgates, as both Prof. Grizwald and a young Fumblemore expressed interest in finding more Gates built by the ancient civilzation