It was Crucio, the Cruciatus Curse (from Bellatrix Lestrange). Bellatrix also inflicted a minor slit on her throat when she was unconscious.
'“What else did you take, what else? ANSWER ME! CRUCIO!”
Hermione’s screams echoed off the walls upstairs.'
'Bellatrix was supporting Hermione, who seemed to be unconscious, and was holding her short silver knife to Hermione’s throat...
“I said, drop them!” she screeched, pressing the blade into Hermione’s throat: Harry saw beads of blood appear there.'