Quietus

"Niccolucio, a young Florentine Carthusian monk, leads a devout life until the Black Death kills all of his brothers, leaving him alone and filled with doubt. Habidah, an anthropologist from another universe racked by plague, is overwhelmed by the suffering. Unable to maintain her observer neutrality, she saves Niccolucio from the brink of death. Habidah discovers that neither her home's plague nor her assignment on Niccolucio's world are as she's been led to believe. Suddenly the pair are drawn into a worlds-spanning conspiracy to topple an empire larger than the human imagination can contain." -- Goodreads.