It's not "the gang goes to Avernus" though. It's Karlach and Wyll (if you made specific choices in his story) go to Avernus, and Tav, but only if Tav romanced Karlach.
Don't have to romance her to go as gang (Karlach, Wyll and Tav). Think I had <40 approval with her too, though maxed out with Wyll (
she was very angry that I forgot to bring her to Gortash
).
I was romancing Shadowheart and when I picked that Karlach ending option, I just left her after making her
disenchant her parents for a bugged cure
, so no final chat with her. Reloaded quickly after to have only Wyll go.
