Invisible Stalkers are creatures of solidified air native to the Elemental Plane of Air. They are akin to Air Elementals and like their kin can be summoned to the Prime Material Plane by mages to do their bidding.
Like their name suggests, these creatures are able to remain invisible at all times, even while performing physical attacks and highly resistant to missile attacks. They are unable to cast spells.
As elementals, Invisible Stalkers cannot be killed on the Prime Material Plane, but rather destroyed and banished back to the Elemental Plane of Air.