Gromm is a male Nord who resides in Bruma and is the manservant of Baenlin. in the Dark Brotherhood quest Accidents Happen it is your job to kill Baenlin. One of the things you'll have to do to avoid the bonus is to sneak past Gromm undetected, which is not too difficult.
 Unique Dialogue
"The Master doesn't like visitors. Leave this house. Now. If you're still here in 20 seconds I'll be forced to crush you."