Lorkhan

Lorkhan, also known as Shor, Lorkhaj, Shezarr and Sep, is most directly responsible for the existence of Nirn.

According to legend, Lorkhan died long ago, giving his life to create Nirn. As such, he has not had much direct influence over events of the Third Era, though he was indirectly responsible for at least three major near-disasters in the last decades of the Third Era, and his remains are connected to the disappearance of the Dwemer.

Creation
It is said that Lorkhan was begat by Sithis to go and destroy the universe, after the Aedra who had enslaved everything Sithis had created. It is unknown whether this is true, although the Dark Brotherhood tend to advocate this hypothesis.

History
Lorkhan is one of only two Aedra (the other being Akatosh) to appear in every known mythology on Nirn. However, opinions of him vary dramatically from race to race. Generally speaking, Lorkhan is considered an enemy of the mer, and a patron and hero to men. The Cyrodilic culture calls him Shezarr, The Missing God, reflecting the fact that he alone of the Aedra has truly died and vanished.

In mer mythology, Lorkhan is called The Trickster, and the mer do not look kindly on him. They blame Lorkhan for tricking the other Aedra into giving up their divinity. This includes those who would become the Ehlnofey and thus the Altmer, who now blame Lorkhan directly for their mortality. The Dunmer have a (very slightly) less venomous view of Lorkhan. Since they believe that their mortality is a test of their strength, and that they are ultimately destined to shed their mortality, Lorkhan is simply one of the obstacles the Dunmer have to overcome.

Men, however, do not believe themselves to be descended from the Aedra. They believe themselves to be created from nothing by Lorkhan, and thus owe their entire existence to him. In their opinion, Lorkhan is the hero of mankind, and they object to his mistreatment by other races.

Lorkhan's Heart
Even more so than the Amulet of Kings, the most influential legacy of Lorkhan's existence was his Heart. Altmer legend details the anger the remaining Aedra had towards Lorkhan once Nirn had been created. As punishment, Trinimac tore out his heart, intending to destroy it. However, the Aedra were present on Nirn, and Nirn itself was the remnants of Lorkhan's body. Thus, his Heart was literally the Heart of the World, and could not be destroyed. Instead, Auri-El flung the heart down to Nirn, intending to bury it forever.

The Heart did not remain hidden, however. During the First Era, while excavating caverns under Red Mountain in Morrowind, the Dwemer located Lorkhan's Heart. At the time, the Dwemer were pursuing the goal of attaining divinity through the combined effort of magic and science, and their High Priest Kagrenac believed that Lorkhan's Heart was the key.

Kagrenac proceeded to build a huge mechanical being, a golem named Numidium, which he intended to power with Lorkhan's Heart. Before he could do so, however, the Chimer attacked the Dwemer, a battle which resulted in the disappearance of the Dwemer. However, Numidium survived its creator's disappearance, and would play a key role in many future events.

The first real use of Numidium came during the late Second Era, when it was loaned by Vivec to Tiber Septim to aid in conquering Tamriel. At this point, however, Vivec and the Tribunal were keeping close control over the Heart of Lorkhan, so another power source was used for the golem (specifically, the soul of Imperial Battlemage Zurin Arctus). The golem would reappear late in the Third Era, hundreds of years later, as Arctus reappears and tries to use the golem to exact revenge. The final encounter with the Heart of Lorkhan came during the Dagoth Ur crisis. It was here that the true power of the Heart was revealed, essentially validating many of Kagrenac's claims. After the defeat of the Dwemer, the Tribunal had used the Heart to achieve near-divine status. Contact with the Heart granted them immortality, at least as long as they maintained frequent contact. It also granted them legendary powers; Vivec flooded all of Morrowind, supposedly granting the entire Dunmer race water breathing ability first, to fend off an invasion.

Indoril Nerevar was the Chimer general responsible for the attack and ultimate defeat of the Dwemer at Red Mountain. When the battle had concluded Nerevar ordered Dagoth Ur to remain behind and guard the Heart, as he and the Tribunal (then mortal Chimer) found a plan on what to do with the artifact. When they returned they found that Dagoth had become insane from tapping into the Heart's power. A battle broke out between Nerevar and the Tribunal against Dagoth Ur. Dagoth eventually lost the duel which ended with Nerevar being mortally wounded. This is where reports differ. One legend states that after finding the Tribunal and making them swear not to use the tools of Kagrenac on the Heart, they murdered Nerevar. Another report simply says that Nerevar died after making them swear to Azura, after which they used the tools, against their oath, and became the living gods of Morrowind (the act that caused Azura to turn the Chimer race into the Dunmer). It was only hundreds of years later that the Nerevarine, Nerevar reborn, was able to destroy the Heart, after the second rise of Dagoth Ur. This caused the Tribunal to lose their immortality as well, simply making them incredibly powerful mortals. After losing her immortality though, Almalexia killed Sotha Sil and was, herself, killed by the Nerevarine. That event led to Vivec being the only member of the Tribunal alive during the Third Era and ended the influence that the Heart had on men and mer.

Lorkhan and Daedra
There are several comments made in that are rather strange and contradictory. Several statements made by Mankar Camoran in his Paradise point out that Lorkhan did make Tamriel, but Lorkhan was lost for an unknown reason. Mankar Camoran also states that Tamriel is not a realm for the mortals nor common folk, but is actually another realm of Oblivion. He says that it is an Oblivion-realm that once belonged to Mehrunes Dagon, the Daedric Prince of Destruction, Change, Energy, and Ambition. Mankar Camoran explains that Lorkhan, having loved his created world so much, was unwilling to let the Prince of Destruction claim it. Mehrunes Dagon is not actually invading Tamriel, Mankar Camoran says, but rather is coming to reclaim his long-lost world. Of course, Mankar may not be a reliable source in this respect.

Trivia

 * During the events of, the Last Dragonborn encounters a mage named Septimus Signus, living in the ice fields north of Winterhold. Speaking with Septimus will begin the quest Discerning the Transmundane, as the Dragonborn seeks to help Septimus open an ancient Dwemer lockbox. When asked what is inside the box, Septimus goes on to say he believes the Heart of Lorkhan to be inside. It is revealed later that the Heart is not inside, but the Oghma Infinium, the tome of esoteric knowledge created by Hermaeus Mora.
 * In Riften's Mistveil Keep, one can find in Wylandriah's chambers a letter from Mirabelle Ervine at the College of Winterhold. In the letter, Mirabelle replies to Wylandriah's request that she does not have a sample of The Heart of Lorkhan for experimentation because no samples exist.