Penta kills do not have to determine you win, i'd say you have a bigger chance of winning the game if all lanes are ahead instead of one person getting a penta.
Though penta kills usually happen at the end of a game.
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http://puu.sh/ofw1Z/2a47f55d4d.jpg
Lucian was dedicated to report Sam hh.
Got carried by my brother, support morgana did most dmg lol.
http://puu.sh/ofGcR/233b737bbf.jpg