Are there deliberate bridges between Princes of the Apocalypse, Out of the Abyss, and Curse of Strahd?

Question

I have a party of 4 level 3 adventurers from a currently running D&D 5e campaign (Forgotten Realms but very sandboxy homebrew pirate adventures - they are all pirates or related professions e.g. smuggler, bounty hunter).

I'm planning to let them fall into one or more of three published Wizards of the Coast (WotC) adventures:

• Curse of Strahd
• Out of the Abyss (OotA)
• Princes of the Apocalypse (PotA)

Are there any potential links between these adventures defined in the published adventures themselves, or otherwise officially defined by WotC, that I can use to keep the segue feeling sandboxy and open to player agency? (I'd prefer links that make sense in-game.)

I remember at least an explicit mention at some point in PotA there are tunnels from one or more of the temple areas into the Underdark, so there's a lead-in for OotA. What about Strahd? Can e.g. the Mists appear deep underground while you are trawling through the Wyrmwrithings?! :-)

Has anyone made a convenient table of potential but explicit links like this between all the public WotC adventures?

Answer 1

Vizeran DeVir, a drow archmage is common to both Princes of the Apocalypse and Out of the Abyss. In the former,

he was the one responsible for creating the 4 Elemental Weapons wielded by the 4 prophets, the main antagonists of the campaign. In the latter, he plays a role in the endgame of the campaign by providing a way for the heroes to destroy/banish the demon lords.

Another thread you could use to link the two campaigns is that the Fane of the Eye in PotA was built by ancient Ghaunadaur worshiping drow. Some say that the demon lord Juiblex is actually an aspect of Ghaunadar. What if instead of

being turned into an ocre jelly, touching the statue in area F19 of the Fan of the Eye (potA p.121), you were teleported to the Oozing Temple (OotA p.34). In any event ou can decide for yourself the relationship of these various entities and how Vizeran DeVir ties in with them.

As for "hidden stairs," as requested in your comment on another answer, I can point to a few between PotA and the Underdark.

1. Fane of the Eye, area F12. Abandoned Mine (PotA p.119) - mine tunnels lead into the Underdark.
2. The Black Geode, area G5. Underdark Entrance (PotA p.136.

Answer 2

So the three official books that readily connect are:

• Lost Mine of Phandelver (D&D Starter Set) (level 1 to 4)
• Hoard of the Dragon Queen / Rise of Tiamat (level 1 to 15)
• Princes of the Apocalypse (level 1/3 to 15)

LMoP and PotA happen literally next door to each other. HotDQ runs over a much larger area that also includes the areas from LMoP and PotA. There are lots of ways to weave those three together depending on what your characters want to do.

In PotA, page 17 includes a series of possible plot hooks. The PotA books could lead in to the underdark, but that's very far in to the book and it kind of defeats the purpose because OotA is supposed to start with the PCs as prisoners. Specifically look here for an entrance into the Underdark.

• PotA:Black Geode:G5 (pg 136)
• PotA:Fane of the Eye:f12 (pg 119)

That stated your goal of weaving those books is going to be tough.

• OotA has a "quest out of the Underdark" component. If you escape the Underdark and never go back, then you could merge into some of the other books, but you're probably missing most of this adventure. (level 1 to 15)
• CoS is explicitly designed with characters trapped in Barovia. Ravenloft is the "Demiplane of dread" and it traps PCs on the plane until they escape. (level 1 to 10).

Really the best you can do with those specific books is probably CoS. If you start in the Forgotten Realms, you can pull level 3 PCs into Barovia (the CoS quest from 1 to 3 is optional). If they escape Barovia at level 10 or 11, you can throw them into PotA at a later part of the book, probably the 3rd set of dungeons. Sadly, that's the best I have.