The Sea of Blood is a dangerous place found near the Wastelands in the Realm of Outworld.
This entire lake was filled with blood. The souls of the dead that are there are put to use with their bodies added to the sea of the blood of the damned. The sea is also bottomless; those who fall in can never escape.
Those that traveled the sea through boat were often challenged by bloody Undead skeletons that boarded their craft and attempted to kill the living.
During their quest in Mortal Kombat: Shaolin Monks, Liu Kang and Kung Lao traveled across it with the younger Sub-Zero.
In Mortal Kombat 11, the stage was located in Netherrealm and it is a place where Kharon lives.
Raiden disposes of the immortal Geras by tying him in a chain and dropping an anchor attached to the chain within the Sea of Blood.
Erron Black's Arcade ending has him dropping Kronika's hourglass into the Sea of Blood, so that no one would ever gain it and use it.