Complex sentence: Dr. Jekyll has just finished with the blue serum with flower sap that turned into a red serum that will separate the evil from the good in a person, but Poole enters and forces him to transform into Mr. Hyde once again by using the blue serum.
Split into simpler sentences: Dr. Jekyll has just finished with a blue serum with flower sap that turned into a red serum which will separate the evil from the good in a person. However, Poole enters and forces him to transform into Mr. Hyde once again by using the blue serum.