Could Strahd do anything with a key to Sigil that would allow him to escape Barovia?

Question

I am DMing the Curse of Strahd adventure. My players' party is a silver dragonborn fighter, a high elf cleric and a tiefling druid. He brought the fighter to Barovia because he's the last living descendant of Argynvost and he wants to wipe out the bloodline completely, and he brought the cleric because [reasons].

Now I've had an idea for the druid, but I'm not sure if I can make it work. As part of her backstory, she's been travelling the planes trying to find a cure for the curse that befell her homeland (which we've decided is Flotsam from the Dragonlance universe, since it's not out of the question that they'd have some kind of "devil's quarter" there for wayward Tieflings). On one of her journeys she got lost in a demiplane, where she attracted the attention of the Lady of Pain due to her having a pretty unique tail, and the Lady sent an emissary to offer her passage to and a key for Sigil in exchange for half the tail, which she accepted.

Obviously the key won't work in Barovia due to the rules on getting to other planes, which is why Strahd can't leave as well (and the druid tried to use it after realising she'd been transported and it didn't work), but obviously if he could he'd want to in order to escape his endless torment. So I hit upon the idea that the reason he brought the druid to his realm was to offer to restore her ruined wings in exchange for her key (whether he can do this or not or even intends to is a subject for another session) but my question is: is there anything in the 5e RAW that would enable Strahd to make use of a Sigil key to escape Barovia if he had one? I mean DM fiat means I could just say it works, but I'd like to have a way to justify him doing this if that's the way the campaign ends up going (at which point Strahd would escape and start terrorising other planes if they don't go after him).

TL;DR: Could Strahd do anything with a key to Sigil that would allow him to escape Barovia?

Answer 1

Not without a DM fiat which runs contrary to the lore

On page 24, the campaign book for Curse of Strahd says, under "Alterations to Magic":

No spell — not even wish — allows one to escape from Strahd's domain. Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, as do effects that banish a creature to another plane of existence. These restrictions apply to magic items and artifacts that have properties that transport or banish creatures to other planes.

To allow anyone to leave Barovia (per page 208):

Strahd's death grants Barovia a reprieve. The fog that surrounds the land thins, and it no longer harms those who pass through it [...] escape is now possible.

So Strahd is kind of out of luck there :-P

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If you want to dig in the crates a little, the 2e sourcebook on Sigil (In the Cage: A Guide to Sigil) talks about the erratic nature of portals to sigil starting on page 8. There isn't really a good sentence to quote here but the basically states that, while you may have a key to a portal, you need to know where the portal is. It may not connect to the plane you're currently on. Or it might have briefly but then switched to somewhere else:

Temporary portals don't follow any pattern at all: they appear and disappear at a whim.

That said, going just off this older lore, while non-permanent portals are really too random and unstable to use, theoretically, there's a non-zero chance that one could open up right in Strahd's living room.

But then that conflicts with the current 5th edition lore mentioned above. So, in using a DM fiat and saying "What luck! A portal conveniently opens in Barovia and it matches the key that Strahd has come by!" you'd have to tweak the lore of CoS pretty radically.