Recurring Nightmare is a Unit.
How to Get Edit
While attacking with Recurring Nightmare, you want to clear your opponent's board of flying creatures that would block it. Additionally giving it one time buffs such as Beastcaller's Amulet and Ghostform are cards that synergize with it.
While defending against it, you can deal with it by blocking with Flying units, by hitting it with a Fast Spell like Torch or playing down a relevant Ambush unit like Desert Marshal or Phantasmal Presence.