Nidavellir

Nidavellir is a rare case of spherical world among the Throne Worlds - and one of size only slightly larger than that of Old Earth, which is something that only Nidavellir and Furya can say about them. It even circles around a sun (again, like Furya), and appears to have something akin to a stellar system (there are other planets, although all of them completely barren and dead). However, this is where the similarities end.   Its surface does appear to be relatively simple - although it's almost entirely composed of mountains and small, forested valleys. However, that's because the weirdness of this world is mostly located underground. The entire world is filled with tens of thousands of kilometres of caverns and tunnels, with entire (often rather scary) ecosystems finding their niche under the surface. The caverns lead all the way to the very edge of the molten core of the world, It shouldn't be habitable, yet it is - the temperatures will appear pleasant even right next to the core. Ground pressure also doesn't collapse the planet's surface, despite science dictating that it should have.   Then lies the most insane part of this world - Muspell, the Vulcan's fortress and a seat of power. It is best described as fortress-shaped emptiness with floor, walls and ceilings being made of liquid metal within the beating core of the planet, inside of which it is located. The fact that its internal decor appears to be made from massive chunks of diamonds (material very common on such depths, with Earth alone estimated to have had quadrillion tonnes of them), is its least surprising feature.   While it should promptly incinerate all who enter (or who even are in relatively proximity to an entrance leading to it), it in fact does not - but the temperature and pressure gets higher and higher the closer you get to the very epicentre of the Nidavellir, where Vulcan's throne room was once located. Ever since the fall of the Dark Lord, no one has managed to get even relatively close to it.