Complex sentence: However, she encountered the monster and on the evening of November 6th, 1983, Eleven finally made contact with the creature however a gate opened up between the Hawkins Laboratory and The Upside Down dimension and the monster entered the human world.
Split into simpler sentences: However, she encountered the monster and on the evening of November 6th, 1983, Eleven finally made contact with the creature. However a gate opened up between the Hawkins Laboratory and The Upside Down dimension and the monster entered the human world.