A Ruinscar is a chasm, a deep rift within the World of Veil. They're blast zones left from the War of Ruin - areas of incredible magical power seeping across the veil. There are scattered throughout the world. They appear in the story alongside Ataraxis' Crystal, which needs to be fused by the power of the Veil, only accessible via the Ruinscars. The Crystal reacts to the power of the Ruinscar, absorbing the power that it requires.
There is a Ruinscar located within three areas in the world of Veil; the Ruinscar of the Sea is located within the Undersprawl in the Coral Tombs, leftover from when Agmera was smashed in the War of Ruin. The Ruinscar of the Forest]] is located inside the Tangled Growth of the Elder Forest, and the Ruinscar of the Mountains is somewhere within the Frost Reaches, however it's location is currently unknown.