How do I find Queen Endregas?

Solution 1:

You don't have to light the cocoons on fire. Beating them to death with your sword is sufficient. Make sure you open them up first. In order to be able to open them, however, you must have the knowledge about the Endrega. You can accomplish this either by killing three of them or buying the book from the dwarf in the nonhuman district who sells books.

Solution 2:

As StrixVaria already mentioned, you only need to open each cocoon and hit it with your sword. There are two groups of three cocoons each, destroying all cocoons of a group will spawn one of the Queen Endregas.

I've marked the locations of the six Endrega cocoons on a map: