After the voice-acted scene (see child claim: Summoning Alandro Sul Scene), the player is asked by Alvu and/or Simine to guide them to the holy relic Alandro Sul pointed them to.
The relic is a hidden antechamber in the Necrom catacombs. The chamber holds the Foul Murder mural, a painting created during the Red Moment, which cannot be destroyed, instead disappearing and reappearing in another location if someone tries to. The Tribunal finally managed to enchant it to lock it away under Necrom.
When the party enters the chamber, everyone stands goggling at the mural. There is a sound, and all turn around to see the Nerevar bonewalker floating on up to the Nerevarine. The bonewalker touches the Nerevarine. Cutscene happens--like the one that starts the game, it should be a bit jumbled and symbolistic rather than seeing any particular thing, to convey the feeling of the player receiving a lot of information all at once. Then the cutscene ends the player is forcegreeted by one of the NPCs, asking the player WTF just happened, that the player had lost consciousness, and the bonewalker turned to dust.
The player can then turn to Alvu or Simine (whoever was their chosen questgiver), and recount what happened at the Battle of Red Mountain through a series of dialogue choices. This should serve as the player choosing which version of the canon they want to be true for their character, such as: that Nerevar was murdered for the Tribunal's personal gain; that Nerevar was murdered because the Tribunal truly thought that would be best for their country; that Nerevar was killed in battle and the Tribunal made the best of a bad situation; that the Nerevarine is not truly Nerevar and just happened to fulfill the prophecies; or my personal favorite, that perhaps Nerevar asked the Tribunal to do it, seeing glimpses of what would happen thanks to Azura. Other popular fanon versions could be added here as well.
Picking one of these stories (or offering all of them as food for thought) would finish the quest. The questgiver would react to these tellings as befits their personality, including stepping down from their post or going on a murderous rampage. If we wanted to be ambitious, we could also change various pieces of the dialogue all over Morrowind to lend credence to whatever version of the story the player picked.