This Adventure is great to try out LvL 20 with experienced players. I ran this Oneshot twice for different groups and all had a lot of fun. All the Encounters were somewhat on the weakside for a coordinated LvL 20 Party of 5 Players. So I suggest giving your players only 1-2 hourglasses and upgrading a few monsters (Adding the Brass Dragon Mother as an Ancient/Adult Dragon, Giving Shinoh more HP and giving the Divine Horror around 500 HP).

Great supplement book for Descent into Avernus. It fixes a lot of issues with this book and gives the DM a lot of useful information while running the game that. The best part of this product ist the running Avernus as a Sandbox supplement. This makes Avernus a lot of fun to run instead of just running your players through one of two linear adventures. The Bitter Rivals Adventure also fits in very nicely into the sandbox as a source for Soul Coins and information to point your players into the right direction. The monster stat blocks are also useful so you don't have to flip through the Monster Manual and have all the info for an encounter on one page.