Another embark where the werebeast in its beast form is able to attack dwarves but dwarves cannot attack it. They aren't even scared by it hence the "invisible", I watched one run through of this save where a dwarf picked up a dead animal to take to the refuse pile. While the were beast attacked the dwarf it moved erratically, once beast picked a fresh target the dwarf carried on with the corpse hauling job having not been concerned enough to drop its hauled load.