Star Gates

Star Gates is a term for unique legendary portals found throughout the world that utilises one of two different types of portal. Both seem to be artificially created and rely on different materials & operations. Exactly who built either is completely unknown to the majority of Earth's inhabitants. If anything, only the eldest of races would have remotely any idea who built them, and where the gates are.

Utility

It seems that the Egyptian one requires the imprint of specific patterns to it to operate, but exactly how or with what is another matter. The Northern one requires the tampering of specific crystals on a pedestal just in front of it to operate.
The former was discovered by Roman invaders of Egypt about 500 years ago after hearing local legends of a deep thrumming from a part of Luxor, and that cracks had appeared around the structure that contained it. While there they found that it seemed to have been recently used and a pile of still steaming metal slag, and organic material lay just outside it's footprint. They decided to seal it away and local forces have watched over it since.
The latter seemed to have been triggered when a Stone Giant staggered out of the mountainous cave that contained it, howling & mutating at an exponential rate. It kept screaming that there was a gateway to Niflheimr in said cave and that it should never be found. Afterwards Dwarves from the Scandes seized it along with High Elves & Wood Elves. A contingent of Black Elves arrived shortly after at Uppsala and asked for help from the priests there to help them and their kin to protect it as it was likely and ancient gate to Alfheim & Svartalfheim, and didn't want any misuse by outsiders.

Manufacturing

No-one knows the manufacturing process of either. The Egyptian gate seems to be made of metallic materials not seen anywhere on Earth. The Northern gate is made of a strange bonelike material that appears more grown than built in a forge.

Social Impact

Most don't know anything of either, even peoples living near it due to misinformation, and being heavily guarded. Those that do, have any number of theories about them, and are too terrified to use them if they knew how. Also, with not knowing how to maintain them or build them, they are scared that if wither fell into the wrong hands it could open up portals to other worlds leading to untold damage.

"I've heard of these two gates. According to inscriptions on the structure surrounding the Egyptian one it opens something called a 'Wormhole' from one gate to another based somewhere else in the universe. Exactly where and what at the other end is completely unknown, or is there multiple exit points. Since we can't tell exactly what or how it operates we may never know, and thankfully too.
As for the northern one, even less is known. The priests of Uppsala are the only ones who MAY mention anything about it but even then. They say it is a miracle device with old Elvish script carved or even grown on it. And there's hints that early Dwarvish too in places. It seems to pulse with a lifeforce of it's own, but never activates.
They terrify me personally. Who built either? Is there one set of builders millennia apart? Different builders? Are they still in use on other worlds or different dimensions? What energy do they use? Are they safe to use? Is there something on the other side that should not be met or let loose? We may never know, and that's why I think they should be left well enough alone."
Father Tossil, speaking to the Vatican Librarian of Legendary Antiquities.

Access & Availability

In both cases they are heavily guarded and hidden from view. So much so, no-one has ever truly been able to say they have seen them, let along used them. Bands of warriors, clerics, and mages are rumoured to protect them and have altered the regions they sit in to the point of leaving no physical trait without knowing the exact details of how they done it.

Complexity

Both are so complex in operation none can operate either, and since they are so different in use, if one were cracked, the other still wouldn't be useable.

Discovery

Two such separate gates are known, or at least rumoured, to exist. The most well-known is rumoured to be guarded & hidden from view at Luxor in Egypt. The second is rumoured to be in the Scandes mountains due west of Uppsala in Sweden.