Magonia is the name of the cloud realm whence felonious aerial sailors were said to have come, according to commonly-held beliefs denounced in the polemical treatise by Carolingian bishop Agobard of Lyon in 815, where he argues against weather magic.
In his treatise Agobard complains that in his region it is widely believed that there is a land called Magonia whose inhabitants travel the clouds in ships and work with Frankish tempestarii ("tempest-raisers" or weather-magi) to steal grain from the fields during (magically raised) storms.
But we have seen and heard of many people overcome with so much foolishness, made crazy by so much stupidity, that they believe and say that there is a certain region, which is called Magonia, from which ships come in the clouds.
Among those so blinded with profound stupidity that they believe these things could happen we have seen many people in a kind of meeting, exhibiting four captives, three men and one woman, as if they had fallen from these very ships.
The life of Agobard in Ebert's Allgemeine Geschichte der Literatur des Mittelalters im Abendlande (1880), Band II., is still the best one to consult.