an ominous host of barbarians

This is not Gibbon, but Tacitus cited by Gibbon, describing the appearance in combat of the Arii:

“Their shields are black, their bodies are painted black. They choose for the combat the darkest hour of the night. Their host advances, covered as it were with a funeral shade; nor do they often find an enemy capable of sustaining so strange and infernal an aspect. Of all our senses, the eyes are the first vanquished in battle.”