The Subterranean Sun is a mini-biome added by the Gensokyo mod. It can be found at the far-right (Eastern) side of the Underworld and consists of a large, circular empty space, with a sun consisting of Stellar Plasma blocks at its center.
Hell Ravens spawn exclusively within this mini-biome. In addition, it is the only place where Utsuho Reiuji can be summoned, assuming that the world was generated with the Gensokyo Mod enabled. If this is not the case, and the Subterranean Sun does not exist, Utsuho can be summoned anywhere in the Underworld.
Players within the mini-biome will continously have the On Fire! debuff applied to them. In addition, flying too close to the sun inflicts the Vaporizing debuff, which causes the player to lose 100 health per second.