real hemlock
Just finished a run with Kaali's Scythe. Fought and killed Kaali with said scythe. Next mission for Kaali in Book of Heads was to kill Kaali with her weapon. Award was not granted which may be based on the fact that she was poisoned and might have died from the poison damage. All the same, it should have counted, since I had the weapon for the quest and I killed the boss with the weapon.
I did kill Altan for the third time and get his Book of Heads reward for the third boss kill, so that may have interfered with it somehow, but I wouldn't know what to look for in the files for that. Will attach logs below.