Eldritch Sea is a sea of monsters, and creatures of many kinds, that constantly pose a danger. It is believed that the reason the worlds shattered was because of a being of great magnitude reaching almost as far as the apparently infinite Eldritch Sea.
The Eldritch Sea is not filled only by shattered worlds and creatures from the beyond, dangers of the Eldritch Sea do not originate from there, as with a multitude of worlds many have found different ways to survive, like stealing Remembrance from passing Nimbus Ship, integrating with a Core of a Beacon or even succumb to the Eldritch Sea and become part of it.