Complex sentence: The object of the games is to collect six different coloured "Keystones" and thus beat the host, most games in the series the "Gatekeeper" is the host but other characters have "step in" and took his place.
Split into simpler sentences: The object of the games is to collect six different coloured "Keystones" and thus beat the host. For most games in the series, the "Gatekeeper" is the host but other characters have "stepped in" and took his place.