The Shadow Plane, also called the Underworld, is the home to demons and Shadow Magic, ruled by the Forgotten One. Utilizing a Dark Crystal unleashes a powerful demon from the plane, that grants Gabriel a vast amount of power to crushes his opponents with. It's very useful against the Titan battles, since you can destroy their runes in one hit after having used the Dark Crystal. They also serve you great in regular boss battles, but not against Satan, because he is resistant to it.
The Forgotten One was released from the Shadow Plane by the Bernhard family, and sealed within it by the leaders of the Brotherhood of Light. After the deaths of the Lords of Shadow, the Forgotten One sought to free himself from his prison, but he was defeated and killed by Gabriel Belmont after the former brotherhood knight stole his powers.
Sometime before 1046, prior to Gabriel's journey, the doors to the Shadow Plane were opened, and several demons, including the Shadow Crawlers and the Daemon Lord, were released from the Shadow Plane. Gabriel was dispatched to kill them and imprison the Daemon Lord, which he did so successfully.
The metal composing the Dark Pain was found in the Shadow Plane, and its experimentation resulted in the creation of the weapon.
It is currently unknown, though very possible, that the Shadow Plane is the Judeo-Christian Hell.