Complex sentence: The only manuscript of the "Tale", claimed to be dated to the 15th century, was discovered in 1795, in the library of a Yaroslavl monastery, where the first library and school in Russia had been established back in the 12th century.
Split into simpler sentences: The only manuscript of the "Tale", claimed to be dated to the 15th century, was discovered in 1795, in the library of a Yaroslavl monastery. The first library and school in Russia had been established here in the 12th century.