Name: ghost
Difficulty: 12
Base level: 10
Base experience: 216
Speed: 3
Base AC: -5
Base MR: 50
Alignment: -5
Frequency: no random generation
Genocidable: No
    Touch: 1d1
Weight: 1450
Nutritional value: 0 (but leaves no corpse)
Size: medium
Resistances: cold, sleep, disintegration, poison, petrification
Resistances conveyed by eating: None

A ghost can fly/float, and phase through solid rock. Due to its unusual body chemistry, It has no need to breathe. It is humanoid. It has an insubstantial body.

And now the souls of the dead who had gone below came swarming up from Erebus-- fresh brides, unmarried youths, old men with life's long suffering behind them, tender young girls still nursing this first anguish in their hearts, and a great throng of warriors killed in battle, their spear-wounds gaping yet and all their armour stained with blood. From this multitude of souls, as they fluttered to and fro by the trench, there came a moaning that was horrible to hear. Panic drained the blood from my cheeks.
The Odyssey, (chapter Lambda), by Homer