Leviathan Horde II
The leviathan horde is not natural which is why numerous stealthy fighters of the merfolk kingdom are tasked to observe the incoming horde. It might be natural if one species only attacks but a total of five different leviathan species are going to attack them.
The merfolk prayed that those ocean giants will just pass through, but their worries became real when the leviathans rammed their bodies against the barrier. As soon as the royal family is informed of the leviathan horde, numerous merfolk magicians immediately raised the barrier to protect Atlantis.
The royal family knew that the barrier will not be able to hold forever as it cannot withstand the attacks of twenty leviathans. It might be able to hold off about five at most but twenty just meant untold catastrophe. The merfolk residents are even told to evacuate using the hidden passageways that they have created.