Nerevar's Tribunal council becoming living gods is one of the most bizarre phenomena in all of the Elder Scrolls lore. Morrowind is many players' favorite Elder Scrolls game, and that's likely partly because of the complex events that occur both during and before the game's main story. Prior to Skyrim and Morrowind, two factions living in Morrowind, the Chimer and Dwemer, waged war over a precious stone known as The Heart of Lorkhan. This Aedric artifact was hidden deep within Morrowind's Red Mountain, and it contained a power that granted immortality; however, it was inaccessible until Kagrenac, a master engineer and high priest of the Dwemer, took it upon himself to study and craft the tools needed to access the stone's power.
Many believe that Kagrenac's tampering with the artifact is the reason the Dwemer aren't in any Elder Scrolls games, although it's not certain why they disappeared from Tamriel altogether. After vanishing, the Chimer became the primary race in Morrowind, with Nerevar as their champion. His inner Tribunal - Vivec, Almalexia, and Solthal Sil - concluded that Kagrenac's tools should be put away, never to be used again. After Nerevar's death, however, the Tribunal went against his wishes.
Morrowind's Red Mountain housed the godly powers of The Heart of Lorkhan that Vivec, Almalexia, and Solthal Sil used to god-like beings in The Elder Scrolls. In defiance of their master and tempted by the stone's lure, "The Three" used Kagrenac's tools to gift themselves immortality and rise up over the past gods. After the Dwemer's disappearance, a split formed within the Chimer race, as some thought the old Daedric deities - Azura, Boethiah, and Mephala - had become too cruel, and thus accepted the Tribunal's newly formed godship. Even so, a lot of the Chimer didn't see them as authentic as other members of the Elder Scrolls pantheon of gods and resisted worship of the Tribunal.
Morrowind's Heart of Lorkhan Stone Is Similar To LOTR's Arkenstone
Like the Arkenstone in Tolkien's Middle-earth lore, the Heart of Lorkhan in The Elder Scrolls is a powerful gem-like artifact found deep in a mountain, and it has the ability to make those who see it crave its power. All three Tribunal members, despite being strong-willed, caved to its powerful calling, just as the Dwarven king Thrane did in Middle-earth's lore. With Kagrenac's three tools each transferred some of the stone's power to themselves, resulting in divine nature.
Most Daedra in the Elder Scrolls series are lesser deities that aren't involved with the life of its inhabitants very much. Fortunately for Vivec, Sothal Sil, and Almalexia, Morrowind's Daedra followed suit with not being very active, and, on top of that, were known for being quite harsh at times. "The Three" were able to captivate the minds of many Chimer, turning them into followers and worshipers under a new identiry: Dunmer (Dark Elves). If it wasn't for the lack of interaction from the Deadra and their harsh punishments, the newly formed living gods probably wouldn't have gained much traction and influence over Morrowind. Additionally, the Tribunal still recognized Azura, Boethiah, and Mephala as important figures, but treated them as old and outdated. This would not last, however.
How Morrowind's Living Gods Became Dead Gods
All three living Elder Scrolls gods underestimated the grasp of power that the Heart of Lorkhan had over their lives. It led to their eventual downfall in the year 3E 427, forcing two of the most competitive races in all of the Elder Scrolls into a new era of rebuilding their identity and culture. The destruction of Dagoth Ur and the three living gods are captured in Bethesda's 2002 release, Elder Scrolls: Morrowind. Nonetheless, there's still a lot of mystery around the subject of the Tribunal's lost power.
After Daogoth Ur and the Heart of Lorkhan were destroyed by the Nereverine (Morrowind's protagonist), the Tribunal lost their immortality over time but remained powerful, as pointed out in the YouTube video from FudgeMuppet below. Most evidence supports that the Tribunal eventually returned to their former selves, prior to becoming gods, which makes sense considering Almalexia's murder of Sothal Sil and her own death from the Nereverine.
The Tribunal's downfall from glory isn't all that clear. Vivec's "36 lessons," which helped the Nereverine defeat Dagoth Ur, also stripped the living gods of their powers. Almalexia and Sothal Sil both clearly die in Elder Scrolls: Morrowind, but Vivec's death isn't as confirmed. It's probable that he stepped down from his role because he knew the Tribunal's time was up. Vivec's controversial disappearance becomes even more intriguing because he didn't return from Morrowind to Skyrim after his supposed death, as other characters did. Nevertheless, all three of Morrowind's living gods were reduced to sainthood by the Dunmer and Chimer people. After 3E the Deadra retook their rightful place as the presiding deities of Morrowind.
There are many interpretations of Vivec's situation that support both his death and his survival. Ultimately, there's no way of knowing for sure at this point, but as Elder Scrolls' lore continues to progress with new games, that could change. Vivec could have a reemergence and become an important part of future plots, or Bethesda may confirm the once-living god's death with hidden in-game details. Elder Scrolls 6' official teasers seem to show that it's taking place in High Rock or Hammerfell - possibly both - which wouldn't make much sense for a Vivec return. However, Elder Scrolls 6 could go back before Skyrim, making it a possibility to get more information about the living gods. At the moment, Bethesda is probably weighing the options of what lore to incorporate and keep out of the upcoming game, along with constructing new additions to the history of Tamriel.
Source: YouTube/FudgeMuppet