Complex sentence: However, unknown to Batman and Robin, the Joker had previously discovered that Sheila had previously performed "illegal operations on teenage girls" in Gotham; after a botched procedure resulted in the death of a teen, she was blacklisted as a medical practitioner.
Split into simpler sentences: However, unknown to Batman and Robin, the Joker is aware that Sheila had previously performed "illegal operations on teenage girls" in Gotham. After one resulted in the death of a teen, she was blacklisted as a medical practitioner.