The Sea of Souls is a safe repository for souls in the Underworld. The living sea swirls with the spirits of those who once walked the mortal realm.
Background[]
The spirits of the newly dead, who have died at peace with their lives, and allowed to enter the Underworld, end their journey at the Hall of the Lord of the Dead. Here they have an audience with Samhain and are then immersed in the Sea of Souls: a safe repository where souls are greeted by ultimate knowledge and are prepared for the next stage of the afterlife. The exact purpose of the souls' audience with Samhain is unclear but has something to do with their judgment.
Once in the Sea the Souls, they enter a trance-like state in their purpose and are uninterested in anything going on around them.
The Lord of the Dead, Samhain's throne, is surrounded by the Sea. A narrow path suspends over the Sea, leading from the Living Gate to his throne.
It is unclear where they go after leaving the Sea of Souls for the next stage of the afterlife. It’s possible they, depending on how they are judged, may end up in some hellish realm such as Hell, Hades, the Abyss, or some heavenly realm.
Behind the scenes[]
Both the Realm of the Dead and Sea of Souls act in the same role as Limbo, in that souls are held there until they can move onto their next stage of the afterlife.
Only the peaceful dead can be placed in the Sea of Souls. Those with unfinished business in the world, and evil souls, are prevented from moving on; forced to stay on the Surface Region until either peace is brought to them, or they become the restless dead that roam the surface aimlessly.