[MENTION=54629]pukunui[/MENTION]: In my high level reboot I kicked it off with the quest from the Dwarven King (name escapes me at the mo

). I didn't bother cluttering the party up with an army (and they were 15th level when we started). The quest is - clean out the Underdark of Demon Lords before they find a way through to the surface.
My rough outline is:
Head to Gravenhollow to gain more intelligence on the threat (a guide leads the way). There they meet a drow wizard (Vizeran) who tells them of a amulet locked in the maze engine that will guide them to the Demon Lords (kind of like a compass).
They then head off to the Labyrinth, stumble upon Yeenoghu by chance (deal with him and his gnoll hoard - a fun encounter it turned out with an epic finale conjured by one of the players!), then find the Maze Engine and recover the amulet. That gets them to Level 16
The amulet has lead them to Neverlight Grove where they've stopped the Fetid Wedding and banished Zuggtmoy back to the Abyss. They're currently dealing with the fallout from that as the Myconids witnessed the battle and are a bit unhappy about it

. Get to Level 17
Next they'll be heading to Sloopidoop by way of Gracklstugh (I'm going to have them enter via the caverns below Gracklstugh where Droki will become a stealthy guide (if a little mad) through the city, I'm assuming they'll want to get a boat to get to Sloopidoop (but given Demogorgon naturally captains don't want to sail that way much anymore, but there's always greed...). They may end up taking the long way of course. Battle with Demogorgon in Sloopidoop. Level 18
After that it's back across the Darklake to Blingdenstone and dealing with the Pudding King who is really Juiblex in disguise, so there'll be a nice reveal once they think they've taken out the King. Oozes galore! Takes them to Level 19
After that it's to Mantol Derith where they'll encounter Graz'zt and find out that they've been doing his work all along. He wants to replace Lolth as the god of the Drow and Vizeran has helped him (Vizeran is a bit of a misogynist and believes the matriarchy has been bad for the Drow...). He wants to make a deal with the PCs (as they've now proved themselves worthy), help him defeat the remaining Demonlords at Menzobarranzan and show the Drow that Graz'zt is saving the city and Drow (not Lolth) and he'll return to the Abyss once he is sure the Drow have switched their allegiance (Vizeran has set up some kind of false feed for Lolth who thinks everything is hunky dory in Drow-land - like switching the CCTV feed

). (of course he has no real intention of leaving - he wants to set up a palace in Menzobarranzan!). This takes them to Level 20. (If they align with Graz'zt he grants them this level up boon as a sign of his good faith.)
Most likely the PCs will not go along with it and will fight him then and there, but if they do it'll set up the final showdown.
Final showdown: Orcus, Baphomet and Fraz-Urb'luu show up in Menzobarranzan thanks to some device planted by Vizeran (basically something saying that Graz'zt is there taunting them - but he's not yet). They come in and start laying waste to the city, the PCs along with Graz'zt join the fray and Graz'zt tries to show off as much as possible while the PCs take most of the damage.
When the main battle is over, Graz'zt will turn on the PCs (when they've been weakened from the battle) and attempt to take them out too. I've no idea how that'll end up.
If they decide to take on Graz'zt in Mantol Derith, when/if Graz'zt is close to losing he will curse the amulet and turn it into an unremovable beacon that will draw the remaining Demon Lords to the PCs.
That's my outline (and actually writing it out here has helped to clarify a couple of plot points.

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(And to anyone who might accuse me of railroading my players, I look at it as a linear adventure (something they happen to enjoy). I've got stations along the way, sure, but how the players choose to get there and what they do at each station is entirely up to them.

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