Keltham asked Tarnish under Detect Thoughts and also truthspell, if she was confident that her commitment to do Literally Anything in exchange for some mortal finally holding Pharasma to any kind of account, would stand up to increasing her own Intelligence, Wisdom, Splendour.
Tarnish's reply, which in its own way revealed more wisdom than many who detect as Wiser, was approximately, 'lol no how could I possibly know that'.
She's fine waiting on all cognitive enhancement until later, and is just here for the Strength, Dexterity, and Constitution boosts.
They can head back, now, though it would also be okay if Carissa wanted to shop around more and buy more amazing magical items that Tarnish can watch her not enjoy.
Or even magic items that might help Carissa in not destroying the world, if she's got any ideas for that. Tarnish is not very against destroying the world, but she does have dreams of her own; she wants the world to be fixed, in part by her own deeds and acts, so that she can go around ever after being awful to Good people and then telling them she's done more Good than they'll ever do and she wants them to suck it up and take it in partial repayment.