Complex sentence: On October 28, Riot Games selling unique Championship skins for hero characters in the game had grown the prize pool to $5.07 million, making it the largest single prize pool in "League of Legends" history.
Split into simpler sentences: On October 28, Riot Games announced that the prize pool had grown to over $5 million, on the strength of selling unique Championship skins for hero characters in the game. The $5.07 million prize pool as of October 28 is the largest single prize pool in "League of Legends" history.