Aquaman and Mera saved some ships from a hurricane. He then returned to Atlantis hoping to regain the throne he'd left behind. While pleading his case, he learned a scout sub had gone missing in the area where his brother Orm, the Ocean Master, had died. Arthur began to lose control - at one instant, he felt his own mind calling out to him from a great distance.
He lashed out at a passing Russian submarine, causing it damage. Mera had to save the crew. Aboard the main sub of the fleet, a crewman explained to Mera that they had found four ancient artifacts and were investigating their mystic nature. They had come together to form (with one of the scientists) the disturbance which now threatened the ocean floor. Aquaman confronted it, but it told him he was only half a man. He was swallowed up by the entity.
Once inside, Aquaman came face to face with Magus, the scientist, who told him he had become a universe and would swallow up everything else. He was told about his astral self - the part of him that had gone missing since the death of his brother. The artifacts had awakened it and it was swimming back to him.
Aquaman failed the first three tests he was given. He emerged victorious and defeated Magus. By doing so, he was reunited with his other half.