He entered the room and sat in front of me like any normal person would do

 But,

 for some reason when he sat in front of me it didn’t felt like that a human was sitting in front of me

it was something very dangerous very powerful it had the intent to kill someone

then

 he said my lord I was an acquaintance of the late duke he said that if something were to happen to him I shall take care of you so please let me be your privet butler.

I asked him that what is the proof that he is an old acquaintance of my father

Then,

 he put his hand in his pocket and took out an antic watch I said how do you have that? He said the late lord asked me to take it as a proof so that you will believe me that I am an old acquaintance of him