Complex sentence: Seeing the success that the WCW had with their video games and knowing that as they were rising again in 1999 they needed a marketable series of games for their fans, the WWF signed an exclusive contract with THQ to produce their wrestling games.
Split into simpler sentences: Seeing the success that the WCW had with their video games and knowing that as they were rising again in 1999, the WWF decided they needed a marketable series of games for their fans. The WWF signed an exclusive contract with THQ to produce their wrestling games.