The party of 20th level adventurers recently discovered their long running nemesis is on the cusp of summoning an Elder God. Most embarrassingly, his super secret lair is actually in the secret escape tunnel from the hidden castle the Hero Party lives in. (The nemisis' construction company was the lowest bidder)
While the Hero Party was able to kill their nemesis, his death was the final component in the ritual. The entire hero party is fighting against hordes of lesser...things. From Beyond. Don't look at them, you'll go funny and then you'll become useless. Quick, all of you squires run up to the Wizards Lab and grab everything from the Sealed Refrigerator of Secrets and return immediately.
In the Refrigerator is a bunch of different parts from...difficult to identify monsters. None of it looks pleasant. One item in particular stands out. There is a plate with a broken and slightly blackened bone on it that has a sign that says "Under no circumstances is this to be removed from this Refrigerator."
Whatever choice the players make, it is the wrong choice. The Hero Party ignores anything they say, take the items, and either curses them for not bringing everything, or says the squires have doomed them all. Then giant tentacles flood from the portal enveloping the Hero Party and pushing the squires back before they retract into the portal and the portal closes (or doesn't close).