Complex sentence: According to in-game information, an ancient civilization imprisoned Regirock along with Regice and Registeel within caverns they themselves had used to inhabit because they feared the three Pokémon, to which they nevertheless felt they "owed all".
Split into simpler sentences: According to in-game information, an ancient civilization imprisoned Regirock along with Regice and Registeel inside caverns, which the people used to inhabit. This was because because they feared the three Pokémon, to which they nevertheless felt they "owed all".