Mantella

The Mantella was an artifact sought by a multitude of groups in. It also brought about the event known as the Warp in the West. It is a massive green soul gem, intended to be used as the heart of the Numidium, a gigantic golem of Dwemer origin, and a replacement for the Heart of Lorkhan.

Creation
It was created from the soul of the Underking, Zurin Arctus, or depending on what source is believed, King Wulfharth of Skyrim. It is said that that the Mantella was created from the heart of Zurin Arctus who disagreed on the use of Numidium with Tiber Septim. When he attempted to take back his heart Numidium was destroyed, and the Mantella was blown to Aethurius. Tiber Septim would then send out his agents, who would eventually become the Blades, to find the pieces of Numidium and Zurin would become the Underking, sending out his forces to regain his heart so he could finally rest in peace.

Underking
According to the source that states that King Wulfharth is actually the Underking, it is said that he was a behind the scenes adviser to Tiber Septim and that he was betrayed by Tiber when he made peace with the Tribunal and accepted the Numidium, and then abandoned the Empire. Tiber Septim and his Battlemage Zurin Arctus would then to soul trap Wulfharth with the Mantella to be used for the Numidium. In the battle that ensued Wulfharth killed Zurin with his dying breath, and when Zurin became a lich it is suspected Wulfharth possessed Zurin or allowed him to reanimate, and thus became the Underking.