If you are looking for free championship Riven skin codes for LoL (League of Legends, then you have come to the right place... the right place for me to tell you that NONE of those sites that give you a download link for a free code is real. They are all fake and scam you into filling surveys that make the scammers rich. All the comments on those site are by falsified fake accounts trying to convince you into filling the surveys. Do not be fooled into supporting such sites.
Below are the only 3 free League of Legends skins that are available to the public: