Here we go Tiamat (Patreon)

Now, I don't usually mess with gods, but people love fighting Tiamat so here's a couple of her avatars: The Exalted Queen and The Godheads Manifest. 5e usually stops tracking CR after 30, but for her I made an extended CR table so you could have a decent idea of how she might stack up to your party; at CR 41 nothing other than a well-prepared T4 party should be able to make a serious go at her.

I also briefly detailed out her lair, so if you're running an Avernian adventure and your players randomly ask to take a peek in there you've got something to show them, and some holy days of Tiamat, so you can mark the passage of time with reminders that Tiamat is always watching.

Speaking of always watching, when Tiamat's mad (and she's always mad) she sends one of these extra-special boys who don't have to follow the rules of hell because their boss isn't actually a devil.

Avatars of Tiamat

Abishai