A crystal portal is an unnatural, magical formation created as a doorway between the world of the living and the dead. Under normal circumstances, of course, individuals do not travel to the Underworld unless they have died, and such a journey is one-way. In addition, their soul must accept their fate, or they become a mindless undead, such as a skeleton or a ghoul. These special portals circumvented the normal gateway, allowing Shadows to enter the world of the living.
Risen 3 EditThe Isle of the Dead was the only known location where someone still living could pass through to the Underworld. Doing so required a special ceremony and potion, and was only available if the guardian of the dead allowed it. Even with these preparations, it is possible that a living person could become permanently trapped on the other side.
When the Titan Lord of Death awoke in Risen 3, however, he used much of his existing power to force crystal portals to form all across the Southern Seas. From these sprang hellhounds, Shadow Lords, soul eaters, and many other nightmares, invading the world of the living. Innumerable deaths resulted, along with many cases of psychological amplification (such as magnifying greed among pirates) or even damage (like Commandant Sebastiano's insanity). The influence of the Shadows even began converting people and corpses into minions.
With the aid of Eldric the Druid, the Son of Steelbeard learns an incantation to destroy crystal portals. Doing so stopped any further egress between the worlds, weakening Nekroloth's hold on humanity. The final set of portals was found on Skull Island. It appears the titan may have possessed the ability to move between individual portals within his lair, rather than only going between the worlds of the living and the dead. Another possibility is that the portals there were either not actually crystal portals or were a special type, with different function.