Complex sentence: Arthur fights Black Manta, who reveals his plan to have manipulated Ocean Master and will soon get kill him and take over; just as he explains this, he doesn't realize Arthur calling a great white shark to attack him killing Black Manta.
Split into simpler sentences: Arthur fights Black Manta, who reveals his plan to manipulate Ocean Master and kill him when the time is right. While Black Manta is revealing his plan, Arthur calls a great white shark to attack him, killing Black Manta.