Complex sentence: After Silas drains her of all of her blood as it became the cure she returned to life; however the 500 years she's existed now catch up with her and she began to descend into old age extremely quickly.
Split into simpler sentences: After Silas drains her of all of her blood as it became the cure she returned to life. However, the 500 years she's existed now catch up with her, and she begins to descend into old age extremely quickly.