Why is the skin called Arbiter Vel'Koz

It's a StarCraft reference.

Because he is the source of Arclight. He spews the stuff everywhere.

arclight is a special skin for varus because it shows varus before his corruption(or righter after it before it showed outer signs at least)

Arbiter vel'koz has a more....mechanical and abstract appearance compared to him(or the justicar skin line for that matter),he kinda looks like some abstract mechanical angel(which is why he reminds me of the bayonetta angels that much)

Well, it's not really Arclight (since he appears to be made of light) or Justicar (that makes no sense to me), is it? Plus, Arbiter sounds pretty damn cool and more fitting for a highly intelligent but less of a fighter being like Vel'Koz.

That and it's a nice reference.

Because Riot has been playing a lot of Starcraft lately cough Reksai cough Zz rot Portal (formerly named Void Gate) cough

Keith David should have voiced Velcok

Well they now renamed the skin into arclight vel'koz ^^

Lets see if the name sticks though.