The full details of Ivy's early history are unknown, but she and Batman have clearly fought before; Ivy had hypnotized at least ten people into worshiping her sexually and then killed them with a toxic kiss.
After being brought back to life by Alfred Pennyworth, Batman descended upon Ivy's lair; his presence affected her plants, causing them to wither and die. Batman then gave her the kiss she always wanted (but in a different way, however, as her poison kiss was unable to affect him now that he was a vampire). He tore her throat open with his teeth and drained her of her blood, commenting afterwards that he could only want her while in the darkness and decay of corruption.
He then subsequently decapitated her in order to prevent her from returning as a vampire and dropped her severed head off at the police department.