Complex sentence: It was in the new theater that Stanislaw Moniuszko's two best-known operas received their premieres: the complete version of "Halka" (1858), and "The Haunted Manor" (1865).
Split into simpler sentences: It was in the new theater that Stanislaw Moniuszko's two best-known operas received their premieres. They were the complete version of "Halka" (1858) and "The Haunted Manor" (1865).