Dark Nebula

A Dark Nebula is a phenomenon which occurs in high altitudes in seemingly random locations regardless of the season. It is a faint yet thick poisonous cloud that is hard to discern from a regular cloud during the day. During the night, Dark Nebulae have been reported to be deep purple under the moonlight. It is classified as Rarely Observed in the Open Blue.

Inside a Dark Nebula [?]
So far, only air-tight, iron-forged ANF aircrafts can fly through Dark Nebulae unscathed. The Marine Charting Committee has dispatched this kind of ships in their attempt to discover what is inside a Dark Nebula, but the results of their research have never been published. The Committee has only marked the locations a Dark Nebula has been seen to form, but those locations do not seem to correlate. Fliers all around the world have been only advised to be careful about what clouds they choose to fly through.