This is obviously off-topic for the thread, but I...
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and SPOILERS for Out of the Abyss
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...just came back from the final session of Out of the Abyss, where the heroes first battled Graz'zt (or rather, his ritual - see the final chapter of the AL scenario "Assault on Maerimydra") and then caught up to Demogorgon just as he broke Orcus' over his knee.
The most important tweak visavi the official stats was to give Demogorgon two actions, two initiative counts. Another rather big thing is auto-grapple on tentacle hit.
Each head could take the Multiattack action to make two tentacles attacks, so four tentacles maximum; but most rounds there was tentacles plus gaze or tentacles plus spell.
And legendary actions - in addition to tailswipe and the legendary gaze (whatever it was called), I added teleport and "bite and toss", much as your Demogorgon does.
Once he flung the Eldritch Knight some 300 ft, and that was from a height of 160 ft
(Demo teleported way up a huge-ass pillar in the middle of the chamber) so that was an easy calculation of fall damage ("yep, I'd say 20d6"). The player was pleased he took his two levels of Barbarian - changed death to not-even-bloodied (even if that was only because his max hp had been lowered
)
Then Big D jumped on top of the Cleric for two tentacle attacks plus 11d6 falling damage (he himself is immune to non-magical damage, such as houses and stuff). He didn't get to enjoy the grappling action because the party Warlock promptly hurled Demo through hell (that is literally the name of the ability) which released the Cleric and gave them time to stand up and brush off the worst
And then Power Word Stun stole his last action (a previous lucky Feeblemind had forced Hethradiah to spend the last legendary resistance, and it was nighty nighty time for ol' D.
One thing I think I got from your pdf was the idea Demo casts Symbol (as one of his spells) though as a single action, simply by forming his body like the spiraling Y. The exact symbol was a random d8, and the lucky bastards rolled a 3 - thanks to Heroes Feast (I believe) the party was immune to that particular effect!
In the end, I can't say I managed to threaten the party (5 chars, level 16) to an inch of their lives (like any good end-of-level fight really
should go down...
) but at least they didn't laugh Demo in the face and made him cry like a little sissy. Which I must say I do fear the book version could have led to.
What my practical experience shows me that these big guys can often keep more than one or two heroes from doing real damage in a given round (the others can't close in, or have DC 23 Wisdom saves with +1 bonuses to worry about...) so their hit points isn't quite the three-round lockdown it first looked like.
Give them just a few henchmen and the fight suddenly becomes WAY harder (like with Squallocks the spellcasting Vrock and Queen Hledh flanking Graz'zt).
But you knew that already.
The trick is to make the SINGLE bossmonster a match for the entire party all on his own. Giving it better legendaries, and a way to at least act twice otherwise (action surge, two heads, non-trivial bonus action thingies) and most of the remaining stats does work.
For the value of "work" that includes "getting ass-whipped by a party eight-ten levels lower". But still.
Zapp
PS. I believe this was the first and only time I really managed to truly fulfill the 8 encounters and no rest thing. (They broke off their short rest when they saw Demo kill Orcus; justifiably thinking he would have regenerated to max hp if they completed their rest before taking him on)
By the way, 8 encounter days is way too much work. Phew! Now I know for sure the "just have 8 encounters and the game works fine" argument is bollocks. Way too much work to set up the epic conditions required to make them not rest. The game simply must be made to assume shorter working days, since that's how 99% of actual adventuring days go down... I can't be arsed to pull off this epic amount of story and prep just to make the game's basics work.