Avada Kedavra

[barren verge]

I accidentally refused to learn the unforgivable curse, what to do?

[gloomy crypt]

There's a quest called: In the shadow of Fate, do that and talk to Sebastian at the end. Say "We must not turn Sebastian in." Then he'll grant you the offer to learn whatever unforgivable curses you dunno. ^^'

[barren verge]

But if I've already made the choice, is there no way to go back?

[twin dome]

Talk to seb when there is no quests involving him, and ask him to teach a curse

[barren verge]

Where?

he isn't here

[twin dome]

Go inside the undercroft

If he isnt there, wait for him or progress in the story a little bit