The striga's crypt is on the island in the swamp cemetery in Chapter V. It is where Princess Adda's sarcophagus is located. It also hides one of the ancient crypts, Saint Gregory's tomb. Γκέραλτ must go there to retrieve some of the necessary components of Raven's armor.
Just as the witcher arrives at the crypt, he is accosted by Velerad who begs him to go inside and try to find out more about the striga's curse and hopefully cure the princess once and for all. Naturally our hero agrees and sets off to find information and armor. Sadly this is a nasty trick and the burgomeister locks Γκέραλτ inside the crypt, leaving our hero with no choice but to spend the night with the beast.