First of all, i decided to write this build because of my friend.
He likes to play riven, so this ones for you.

Cheers mate.

There is a choice between Merc's boots or Ninja tabi's depending on your enemy team's damage output. If there's alot ad you take ninja's, if there is alot of ap or cc then take merc's.
If there's not too much ap damage, you might wanna swap out the Maw of Malmortius for something you find usefull.