Complex sentence: Myshetsky mentions the date 948 as the date of the first Cossack Varangian Guard on Byzantine service, explaining the name "Cossacks" as Greek misspelling of "Khazars".
Split into simpler sentences: Myshetsky mentions the date 948 as the date of the first Cossack Varangian Guard in Byzantine service. He said that the name "Cossacks" was a Greek misspelling (or transliteration) of "Khazars".