I tacked on the last five adventures of
Scales of War (after running
Palace of the Red Pasha as an immediate follow-up). The overall plot arc went like this: After being thwarted at being able to enter Faerun, and with the help of some rebel devils, Tiamat gained the services of a lich who devised a spell that would allow her to summon Bahamut to the Nine Hells. She then killed him and absorbed (most of) his essence, allowing herself to manifest outside of her prison. The PCs had to resurrect Bahamut, but he came back in a weakened state because Tiamat had absorbed so much of his essence. So they also had to obtain a magical artifact, the Arrow of Fate. When they fought Tiamat for the final time, it wasn't to kill her but to shoot her with the Arrow of Fate, releasing Bahamut's strength back to himself and making her unable to manifest outside the Nine Hells again, so effectively re-imprisoning her. (I'm just not a fan of god-killing campaigns; makes the gods seem way too small, IMVHO.)
There's an ongoing 5E conversion of "Scales of War" at this blog:
https://5escalesofwar.wordpress.com/ . I had to do the last few adventure conversions myself, though, as I got ahead of the blog. For the final adventure, instead of regular chromatic ancient dragons, I used what my players called "off-brand dragons" from a variety of third-party sources, notably the
Tome of Beasts. I cut out the whole "brood mother" thing because it didn't make sense with how I'd reworked the plot.
"Last Breath of the Dragon Queen" (the final adventure of the series) is in
Dungeon magazine, issue 175. I started my sequence (after a bridging adventure) with
Grasp of the Mantled Citadel, which is in
Dungeon 171.
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I've also talked to someone who re-worked The Doomvault
Tales from the Yawning Portal) as a high-level dungeon and made that the gateway to killing Tiamat, but I don't have many details on that.