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Apparently, in 968, the future Byzantine emperors Basil II and Constantine VIII were betrothed to daughters of the Bulgarian Tsar Boris II. But then a year later, Emperor Nikephoros II Phokas was murdered and his successor, John I Tzimiskes, went on to invade Bulgaria, capture Boris and his brother, Roman, castrating the latter and formally divesting the former of his title and authority.

IOTL, Basil never married and Constantine married a Greek noblewoman named Helena, producing three daughters, including the Empresses Zoe and Theodora. But what might have happened if the planned joint-marriages with Boris' daughters had gone through, with or without Nikephoros' death?
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