While in the area, you take 35 damage and are interrupted by the falling pieces of rubble.
This Effect can be encountered in the Arborstone (mission), after one player has picked up the Urn of Saint Viktor inside the entrance to the Cathedral zu Heltzer. While cathedral rubble will fall on you throughout the mission after this point, it occurs much more frequently when in an area displaying the effect icon.
The damage shouldn't be a big problem and can usually be absorbed. Much more severe is the interrupt effect, which applies not only to spells but to any skill that needs time for activation, even attacks. This effectively renders most skills useless while under the effect.