Complex sentence: After the Justice League escaped from the abyssal plain, Aquaman realizes Ocean Master is not using the relic scepter to sink the city and the Trench can only be commanded with the relic scepter that someone has used to manipulate the Atlantean war.
Split into simpler sentences: After the Justice League escape from the abyssal plain, Aquaman realizes Ocean Master is not using the relic scepter to sink the city. Since the Trench can only be commanded with the relic scepter, someone else must be using it to manipulate the Atlantean war.