Complex sentence: The company later decided that an original story created by the Lego Group itself would be more ideal on their behalf, with their first attempts ending up as the themes Slizer/Throwbots and RoboRiders, released in 1999 and 2000 respectively.
Split into simpler sentences: The company later decided that an original story created by the Lego Group itself would be more ideal on their behalf. Their first attempt of this resulted in the themes Slizer/Throwbots and RoboRiders released in 1999 and 2000 respectively.