Arcade is a series of alternate future/universe skins in League of Legends, featuring champions digitized into an arcade setting.
Arcade Miss Fortune, 
Arcade Riven, and 
Arcade Sona are the Holy Gaming Trinity, the goddesses of RGB destined to "save the game", while 
Arcade Ahri, 
Arcade Corki, 
Arcade Ezreal are more rounded "players" who have only just arrived in the game-verse. Another examples are 
Final Boss Veigar and 
Battle Boss Blitzcrank, with Veigar being a being on his own level - messing with the game code at will, fully conscious of where he is and what he can do, while Blitzcrank charges around driven solely by a copy of Poro Roundup.[1]
Arcade officially encompasses three skin lines: Arcade Heroes, Arcade Battle Bosses, and Demacia Vice.