I like to tell whoever will listen that Sauron, the balrogs, the “big” spider Shelob, etc. were really just small-fry compared to their progenitors - Melkor & Ungoliant.
To tell the story of the Silmarillian - from the beginning would be awesome, and epic - a LOTR ‘Clash of the Titans’ if you will...
And then I found this:
https://www.youtube.com/watch?v=0N45ijMZNwc
The Silmarillion is a compendium of decades of writings by Tolkein on the history of ME that were compiled by his son Christopher after his death. There is a ton of source material to draw from.
If they want a GoT type story I think The Fall of Numenor would work best, with Sauron (still in human form at this point) acting as First Minister to Ar-Pharazon, whispering lies to get him to bump off his relatives or send them into exile. Utimately he convinces the king to build and send a massive fleet to try to invade Valinor itself. Needless to say the result is not pretty and does not go well for the poor king. :-)