Complex sentence: It is revealed that he and his leech attack, are what released the virus into the Spencer Mansion and in turn this is what gave Albert Wesker and William Birkin the chance they needed to betray Umbrella.
Split into simpler sentences: It is revealed that he and his leech attack are what released the virus into the Spencer Mansion. In turn, this is what gave Albert Wesker and William Birkin the chance they needed to betray Umbrella.