Postquam rex audivit Hydram caesam esse, perterritus erat. I can translate the sentence but it is the second clause that I am wondering about. At first glace it looks like a past perfect passive(he had been terrified) but the book I am using almost always puts the verb at the end even a copulating verb so I thought that the clause could be translated as ‘he was terrified’ which also seems to make more sense in this case.
You’re right. “He was terrified” is the right translation here: the perfect participle is used as a predicate adjective.