Complex sentence: Using her as a test subject, there was a 50/50 chance that the cure would either work or kill Diamondback and if it worked, Diamondback had to serve Superia as the second Snapdragon.
Split into simpler sentences: Using her as a test subject, there was a 50/50 chance that the cure would either work or kill Diamondback. As an added stipulation, if the test was succesful, Diamondback had to serve Superia as the second Snapdragon.