This is badly needed. The whole thing really needs a set of cheat sheets for synopsis, key developments, etc. A plot chart would help too.
I read through it yesterday. It's pretty sandboxy. Here's what I remember: SPOILERS
Escape from drow slave pits.
Choose a direction to travel in the underdark, engage in a long term fight from drow hunters, using the travel and exploration rules, have various random encounters and discover strange labrynths, most likely ending up at an underground lake at a Kuo Tuo or a Duergar city but possibly at a Myconid forest. Find out the place is going crazy, maybe see a demon lord.
Return to the surface. Enjoy the sunshine. Get summoned by the Dwarves of Gauntelgrym to lead an expedition of torchbearers into the depths to stop the demon lords.
Visit an underground trading post
Visit a mystical Stone Giant Library
Visit the Tower of a renegade drow wizard who has a PLAN
Search the Underdark for spell components for the ritual to lure all the demonlords together and banish them. Maybe see a demonlord.
Go to a demon lord wedding. Have an astra battle in a rotting skull, then maybe a physical battle with a demon lord.
Go to the Drow Homeland City of Menarezzzoboring to steal stuff.
Summon all the demon lords to a godzilla battle (option to have players play them) then maybe fight the surviving demon lord who is hopefully super messed up.
And I think that's a wrap!