Complex sentence: On December 23,2017, she suffered a second heart attack (massive one), after which she fell into a coma and which left her with what was described as "major brain damage," leading to her death on December 30,2017.
Split into simpler sentences: On December 23,2017, she suffered a second heart attack, after which she fell into a coma. She was left with "major brain damage," leading to her death on December 30,2017.