I have the quest from Renal Bloodscalp to find Mae'var`s treachery, but when I go to Mae'var`s guildhouse I can`t find him. I`ve been up and down the stairs and in-n-out every door and although everybody in the guildhouse says he is down the stairs - he isn't. What am I missing? Do I go through the painted door somehow?

he's downstairs and in the back, behind the prison cell (you can walk between them) and he's back with other dudes torturing some guy

I had the same trouble first time through. Directions from the front door- Go through door to the left- there is a stairway just to your left that is easy to miss- go down. Secret door on north wall downstairs- ta da.