Challenger Ahri

It is legacy so it will be back, but most likely not until the start of the 2016 season in February.

The Challenger skins are apparently a new line of skins that are to be sold at the beginning of each season. If it's brought back It'll most likely be at the beginning of the next season. They could be similar to the Victorious skins and just be a one time thing though. I guess we'll see at the start of next season (Or if a rioter answers your question.)

to be honest challenger skins really shouldnt be a thing. we have enough skins that are on a rotation and the last ahri skin was popstar, which was a very good while ago.

I got it as a self-given mystery gift a while back. :3 I only bought two mystery gifts at the time, got pharaoh Nasus as well.

Next year, most likely. Also, if you're lucky enough at some point, mystery gift can unlock it.