Immediately, though everything else remained as before, dim and dark, the shapes became terribly clear. He was able to see beneath their black wrappings. There were five tall figures: two standing on the lip of the dell, three advancing. In their white faces burned keen and merciless eyes; under their mantles were long grey robes; upon their grey hairs were helms of silver; in their haggard hands were swords of steel. Their eyes fell on him and pierced him, as they rushed towards him. Desperate, he drew his own sword, and it seemed to him that it flickered red, as if it was a firebrand. Two of the figures halted. The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.
The Fellowship of the Ring, by J.R.R. Tolkien

Name: Nazgul
Difficulty: 17
Base level: 13
Base experience: 373
Speed: 12
Base AC: 0
Base MR: 25
Alignment: -17
Frequency: Very rare
Genocidable: Yes
    Projectile as weapon, or if in range, uses melee weapon for 1d4
    Breaths sleep: 2d25 turns
Weight: 1450
Nutritional value: 0 (but leaves no corpse)
Size: medium
Resistances: cold, sleep, poison
Resistances conveyed by eating: None

Due to his unusual body chemistry, A Nazgul has no need to breathe. He is humanoid.

Name: wraith
Difficulty: 8
Base level: 6
Base experience: 123
Speed: 12
Base AC: 4
Base MR: 15
Alignment: -6
Frequency: Quite rare
Genocidable: Yes
    Touch: Drains an experience level
Weight: 0
Nutritional value: 0
Size: medium
Resistances: cold, sleep, poison, petrification
Resistances conveyed by eating: None

A wraith can fly/float. Due to its unusual body chemistry, It has no need to breathe. It is humanoid. It has an insubstantial body.