Heart of Lorkhan

"But when Trinimac and Auriel tried to destroy the Heart of Lorkhan it laughed at them. It said, "This Heart is the heart of the world, for one was made to satisfy the other." So Auriel fastened the thing to an arrow and let it fly long into the sea, where no aspect of the new world may ever find it."

- The Monomyth

The Heart of Lorkhan was an Aedric artifact hidden beneath Red Mountain during the Dawn Era. Its most prevalent appearance was during. The Heart of Lorkhan is known to be the Tower Stone of the Red Mountain.

Dawn Era
The greatest influence the god Lorkhan had upon Nirn was his heart; taken from him by the other gods in their fury of his creation of the world. It was cast down into the depths of Nirn, never meant to be found.

First Era
Sometime in the First Era, the Dwemer discovered the heart while excavating under Red Mountain. Kagrenac, the Chief Tonal Architect and High Priest of the Dwemer began to experiment with the Heart. He created three tools: Keening, Sunder and Wraithguard to work with the heart. He would soon order the construction of the Numidium; a massive mechanical golem. He hoped to use the Heart as a power source for extracting the divine power from the Heart and in the experiments for improving the Dwemer race.

This plan was interrupted by the arrival of Indoril Nerevar, his generals (Vivec, Almalexia, Sotha Sil, and Voryn Dagoth), and an army of Chimer. At length, Nerevar and Dagoth fought the Dwemer king and Kargrenac in the heart chamber beneath Red Mountain. When the Dwemer were bested in combat, Kagrenac used the tools on the heart, and the entire dwemer race vanished.

Nerevar entrusted the tools to Voryn Dagoth (who had requested that the tools be destroyed) and met with his other generals. They agreed to keep the tools but to not use them. Upon returning to the heart chamber; they discovered that Voryn Dagoth had gone mad and refused to relinquish the tools; stating the Nerevar had commanded him to keep them from falling into the wrong hands. Nerevar and Dagoth fought, and Dagoth was driven away, forced to surrender the artifacts.

This turn of events concerned Nerevar and he decided to ask the Daedric Prince Azura for guidance. However, his generals murdered him during the ritual. Azura still came and cursed the Tribunal for their crimes; claiming that she would use her powers to resurrect Nerevar. She then changed the Chimer into the Dunmer.

Then the Tribunal did use the tools on the heart and became living gods; they did many great deeds for their people. Since their deeds worked against the evil nature of the heart, they needed to return to it from time to time to rejuvenate their powers. At some later point, they discovered that Voryn Dagoth (now styling himself as Dagoth Ur) yet lived and had connected himself directly to the heart's power. He drove them away, and on subsequent attempts to recover the heart chamber for themselves, they lost control of Keening and Sunder to him and barely escaped alive, supposedly due to the quick actions of Vivec.

Over time, the Tribunal became weaker and weaker, whilst Dagoth Ur became stronger still. They disappeared from public view, and their influence faded. Their sole attempts at restraining Dagoth Ur (the Ghostfence) consumed much of their remaining power and were largely ineffective at containing the Blight. They also could not prevent Dagoth Ur's forces from flying or tunneling past the Ghostfence.

Third Era
Towards the end of the Third Era, Nerevar was indeed reborn, and after a great many trials, he came face to face with his old ally and broke him. However, while sustained by the heart, Dagoth Ur could not truly die. By using the tools Sunder and Keening on the heart again, without a proper ritual, the Nerevarine severed the powers of the heart from the mortal plane and was able to deliver a final death blow to Dagoth Ur and destroy his great weapon, Akulakhan.

Fourth Era
The Heart is briefly mentioned during the quest, Discerning the Transmundane. When Septimus Signus discovers a Dwemer Lockbox brought in by the sudden flood in Winterhold, he believes the Lockbox contains the Heart of Lorkhan. The Dragonborn found an Elder Scroll and inscribed the encryption into a Lexicon. The Elder Scroll and its knowledge was apparently held in Alftand. The quest may only be started by meeting Septimus Signus at his outpost north of the College of Winterhold. Upon completion of the quest the Dragonborn and Septimus Signus discovered the Lockbox does not contain the Heart but instead the Oghma Infinium.

The Heart was also mentioned as being requested by Wylandriah in her letter to the College of Winterhold.

On Solstheim, there were stones of unknown origin known as Heart Stones that appeared after the eruption of Red Mountain, suggesting they may be somehow related to the Heart.

The Citadels of the Sixth House
Using Keening and Sunder while Wraithguard is equipped, attack the heart until it loses it's power.

Discerning the Transmundane
Septimus Signus believes the heart to be located inside the Dwemer contraption within his outpost.

Trivia

 * Despite being an NPC, the heart cannot be soul trapped. It does have a petty soul, and if you try to soul trap it you get a message saying "Your spell did not get a target."
 * It is important to note that within the game it is realistically impossible to kill the heart outright (unless using cheats). This is because the heart has a great deal of health and ridiculous regenerative powers. Only the Keening and Sunder can banish it. However, if it is destroyed through cheats, the main quest cannot be completed, as the heart's power is still bound to the plane, even though it can no longer be accessed.