Angel Demons!
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After searching the library, Vayne approaches the librarian with a hood to cover his horns and his tail tucked underneath a cloak, asking if there are any books on Angel demons. In particular, there general whereabouts, any sightings in and around Arabel, and the extraction of one of their wings for research purposes
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The librarian, who recognizes a tiefling pretty quickly explains that the books rarely discuss such things and he's not sure it is possible since angels are servants of gods and demons entities of pure chaos and evil. It is the nature of an angel to serve, and the nature of a demon to betray and destroy, he suspects they are opposite beings though there is occasional lore of angels who betray a god and become demons. He suggests searching through the books in the library to see if he can find anything on his own.