Simak, Clifford. Where the Evil Dwells
Divergence: 395 CE
What if: Dragons, fairies, etc, are real. Also, Rome did not split into two empires.
Summary: The appearance of "The Evil" from over the river provides incentive to hold the Roman Empire together in a time of schism (c. 1400).
Published: Ballantine/Del Rey 1982 (0345307704), 1983 (0345295712), 1988 (0345334388); and Mandarin 1989 (0749300795).
Original in: English.
Translation: German by Ulrich Kiesow as Im Land der Drachen, Knaur 1984 (3426057832).
Translation: Portuguese by Eurico Fonseca as Onde Mora o Mal, Livros do Brasil 1985.