The soundtrack. The musical score of Morrowind, by Jeremy Soule, blows any game from the early 2000s out of the water. The moment the main menu opens up, you are greeted to the deep, thunderous beating of a drum- or is that a heart beat? From then, the first notes of the song play, starting simply and softly, but rising in intensity until the player is ready and eager to play. It's no wonder this song is called 'Nerevar Rising-' this is the theme song that made the Elder Scrolls as it is today. Even with every change the later games made, Bethesda wisely chose to keep the melodic arrangement of Morrowind in the later games, albeit changed to suit the tone of the games.
The player character himself, the Nerevarine! Not only do you defeat death itself by curing Corprus disease, becoming The Ageless to say the least, but you also manage to kill at least two gods in the main quests, with the possibility to kill another one and the aspect of an even MORE powerful god, and that's not even mentioning how you unite three Great Houses, unite four disparate Ashlander tribes, and destroy the aspect of a god so undestroyable that actual deities couldn't manage it. The cherry on top? Canonically, Morrowind, Tribunal, and Bloodmoon all take place one year apart. This means that you did all this, on top becoming the leader of all guilds, one great house, the friend to countless organizations, religious or otherwise... in just three years.
Everything about Divayth Fyr. He's become something of an icon to the Elder Scrolls lore community for all of the things that make him awesome. To note:
He's somewhere in the ball park of 4000 years old, making him one of the oldest non-divine beings on Nirn.
He "cures" the Nerevarine of the negative effects of Corpus, leaving him/her as The Ageless and immune to all other disease.
And though it was eventually Jossed, there was a long running theory that he was the hero of The Elder Scrolls Adventures: Battlespire game, based on the fact that he had a number of books and weapons found in that game in his possession.