Answer:
Gathered together.
Explanation:
The word "concentrated" can be used in different ways, be it a verb or an adjective. Its use depends on the need and requirement of the sentence.
As seen in the given passage, it is used to talk about the werewolf population. Considering the sentence form, the word "concentrated" is used as a verb. It most likely means the grouped or gathered nature of the werewolves. In other words, it is used to refer to the werewolves found together in one spot or a particular area in large numbers.
Thus, the correct answer is 'gathered together'.