Hell's Core (ヘルズ・コア Heruzu Koa) is a room located in Cube known as Tartarus's point of rebirth.
Located somewhere in the floating island Cube, Hell's Core is used a laboratory where Demons are born, as was the case of Minerva Orlando, and also reborn, as was the case with Tempesta and Jackal. According to Lamy, this is because of a contract made with the Demon King, making Tartarus an immortal Guild.