The Mantella is a massive green gem created from the soul of the Underking, that serves as the heart of the Numidium, a gigantic brass golem. The Mantella must be placed into the body of the Numidium in order to resurrect it. When Tiber Septim used the Numidium during his conquest of Tamriel, the Underking became infuriated that his stolen heart was being used in such a manner. The Underking, a Shezarrine lich who was confused for both Ysmir Wulfharth and Zurin Arctus, fought to reclaim his heart, but the battle resulted in the destruction of both him and the Numidium, and the Mantella was blasted into Aetherius. Tiber Septim sent out agents to recover the scattered pieces of the Numidium, while the Underking sent his forces to recover the Mantella which contained his life force.
Its location was discovered in 3E 401 by Nulfaga of the Wrothgarian Mountains in High Rock. An agent of the Blades was teleported to the Mantellan Crux, and recovered the Mantella just before the event known as the Warp in the West in 3E 417. During the event, it is believed that the Underking reclaimed his lost heart from the Mantella, giving him the death he had sought for so long.