It is a branch of the facial nerve (CN VII).

Its occipital branch supplies the occipitalis muscle.

As it ascends between the external acoustic meatus and mastoid process it divides into auricular and occipital branches.

[1] The posterior auricular nerve can be tested by contraction of the occipitalis muscle, and by sensation in the auricle.

[3] This article incorporates text in the public domain from page 905 of the 20th edition of Gray's Anatomy (1918)