Brain Slug. I will actually be in Ireland for Halloween but at least this little guy could come with me for a easy Halloween costume.
A person assimilated by a brain slug will be likely to speak in a monotone voice and do everything to further the spreading of brain slugs throughout the universe, but the easiest way to tell a person is infected is by the brain slug sitting on their head. The person assimilated retains awareness of his condition, which Hermes referred to as a "nightmare". The recommended procedure of dealing with a brain slug victim is to act natural and switch to a garlic shampoo. Another way to deny attachment is wearing a helmet. Fortunately, brain slugs are not very intelligent and can be easily removed without harming the affected person, minimizing the risk. Once removed from the afflicted person's head, the person returns immediately to normal.
Pattern from Ravelry