Darth Strider
The Bringer Of Pain
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The temple itself resembles a giant rocky building(material is unknown) about 2/3 of a mile tall and half-mile width. It surrounded by four giant nexuses connected to each side which seem to be a source of temples own gravity field and breathable atmosphere.
Insides indeed look even more strange than its appearance from the distance - a giant labyrinth with absolutely nothing inside. Though there seems to be some weird logic here cause no matter which way you go sooner or later you will come right to the center.
After discovering The Temple shortly after Mandalorian Wars, Sith Empire attempted to identify it's purpose thinking that it might be some sort of weapon but research led to nothing and so the Temple was abandoned. Many Siths later had tried to learn more about it yet failed. It's worth noting that one of them somehow managed to activate mechanisms of the building, effectively turning on some sort of gravity beam and trapping himself inside forever. After that incident temple became possessed by his spirit who couldn't become one with The Force and souls of the ones he killed later.
Without any knowledge about that Darth Strider's team came to The Temple to hide for a while but was trapped as well. However, they managed to make their way to center of the temple and defeat the spirit of the ancient Sith.... Yet had to pay a price being sent into stasis for thousands years by his last suicidal attack. Still, remnants of his power began to fade away as the time passed and now there's almost none left to hold The inquisitor captive...