All those chosen by the Professor should be present outside the keep at the designated time. Tardiness, failure to bring the proper equipment or insubordination will be punished with death.
We shall mount our principle attack on Kaer Morhen with our frightener. Gaining mastery of the keep should not present much of a problem, for we have heard only a handful of witchers remain in residence there. Therefore you must make haste: a 50 orens reward awaits you for every mutant head you sever.
Once we have taken the witchers' secrets, we must separate, in order not to attract attention. Every one of you will then be left to find your own way back to Vizima, where we will reunite.
Please forgive me for my tardy response – I was waiting for Golan Vivaldi to confirm sufficient funds were available to redeem the bill of exchange you provided. Not that I don't trust you, but the gods help those who help themselves, as the Kaedweni saying goes.
Getting straight to your question: Princess Adda's curse could indeed be caused to return. The witcher from Rivia had only a rudimentary knowledge of magic arcana, and so was not able to fully remove the spell, and instead only temporarily disrupted its workings.
For Adda to become a striga once more, one would have to sprinkle her food with three drops of blood from her father, King Foltest, three drops of blood from a wolf and three drops of bile from a freshly-buried woman, one who died having finished one hundred years but not yet having started her hundred and first. Then one must cast the Incepted Triangle over the meal thus prepared. That should do the trick – yet please consider carefully whether you truly wish to carry out such an ignoble deed.
Hoping to do business again with you in the future,