The Kingdom of the Trench is one of the seven kingdoms that splintered off when the ancient Atlantis sank. The Trench that resides there has regressed into savage deep-sea beings. The "Trench" is often understood to mean "Mariana Trench" in the Pacific Ocean, but it is strongly implied to be found in the Mediterranean Sea, somewhere near Sicily, Italy.
In the depths of the Kingdom of the Trench exists a bright and violent maelstrom which is a one-way portal to the Hidden Sea at Earth's core. Only the true heir with the Trident of Atlan can bring people back.
Becoming a solo kingdom
During a test with the Trident of Atlan, Atlantis sunk into the ocean. The kingdom separated into 7 kingdoms, and survivors who did not join the 6 formed the Kingdom of the Trench, the Trench continued to dwell in the darkness and survive as evident by the statue of a Trench King in the Council of Kings, eventually the Trench ceased to coexist with the other Atlanteans due to its inhabitants devolving and becoming savage beasts. Deep within the kingdom, there is a portal to the Hidden Sea in Earth's core where the Trident of Atlan resides, only to be wielded by a worthy king.
When Orvax found out that Atlanna had another son that was half Atlantean and human, he sentenced her to the Trench. However, she made it to the Hidden Sea and survived, and was later saved by Aquaman and Mera.
Reunion with Atlantis
After Aquaman wielded the Trident of Atlan, the Trench were ordered to stop Atlantis from killing the Brine and their king and from attacking the surface world during the War for Atlantis. After the war ended, the Kingdom of the Trench was among the kingdoms that reunited with Atlantis.