Mawloc
Mawlocs are monstrous Tyranids that live almost entirely underground. They are nearly blind, but their sense of touch is extremely acute; they are capable of ducking into and out of combat as well as any human, by sensing the movements of the air around them.
Mawlocs locate prey by detecting regular rhythms, like evil versions of Dune's sandworms. Once they have heard such a signature, they tunnel up to the surface, burst out of the ground, devour anything that fits in their maw (get it?), and smash anything that doesn't, before diving back underground to find their next target. They are used to bypass enemy defenses and attack vital elements of the enemy's organization -- supply convoys, headquarters, artillery batteries, airfields, and other assets hidden well behind the front line. Support units that thought themselves safe from attack may find themselves rudely surprised by the tell-tale rumbling of the Mawloc's approach.