Nightmerish creature

I was aimlessly roaming the streets until the night had arrived. Dimly lit streets, only a few torches on poles all around. Citizens seem to be inside their houses with very few exceptions. The guards are also walking around. I was already around the entrance, the person I met before really was there, waiting for me. He had a mask, his whole body was covered in black clothes, decently thin clothes. But I still couldn't see anything special about him besides his physique, he was a frail person, he didn't have much muscle but I saw that he was surely pretty nimble with most of his movements. He must be a skilled individual, which makes sense since he was working directly under the king.