A group of ravens is called an ‘unkindness’. Most animal grouping names are listed in James Lipton's ‘An Exaltation of Larks’. Lipton's list is substantially based on very old sources. There were manuscript lists of group names in the 15th century, and these lists appeared in some of the first books printed in England.

"Be that word our sign in parting, bird or fiend," I shrieked, upstarting- "Get thee back into the tempest and the Night's Plutonian shore! Leave no black plume as a token of that lie thy soul hath spoken! Leave my loneliness unbroken! - quit the bust above my door! Take thy beak from out my heart, and take thy form from off my door!" Quoth the Raven, "Nevermore."

Edgar Allan Poe ~ The Raven