Ravens, B, are a monster in NetHack most notable for their ability to claw at your eyes, blinding you. Having your eyes covered (or being polymorphed into a form with no eyes) will protect against this; however, most means of accomplishing this will blind you temporarily anyway. Having telepathy is useful, although arguably lenses or a visored helmet are the best all-round method of defending against this monster. This matters if you rely heavily on engraving Elbereth.
Your eyes are not actually clawed out; you are merely blinded for d6 (more) turns.
But the raven, sitting lonely on the placid bust, spoke only
That one word, as if his soul in that one word he did outpour.
Nothing further then he uttered -- not a feather then he fluttered--
Till I scarcely more than muttered, 'other friends have flown before--
On the morrow *he* will leave me, as my hopes have flown before.'
Then the bird said, 'Nevermore.'