Plot to assassinate Albus Dumbledore

These attempts on Albus Dumbledore's life were the events that led up to the Battle of the Astronomy Tower in June of 1997.

First Two Attempts
The first attempt took place in 1996, when Rosmerta Imperiused Katie Bell and made her try and give an opal necklace to Dumbledore. She was thwarted when she got in an arguement with her friend Leanne. Leanne accidentally made her touch it and she was curseed. She was sent to St. Mungo's. The second attempt took place on Ronald Weasley's seventeenth birthday, when an Imperiused Rosmerta gave Horace Slughorn mead to give to Dumbledore. Unknown to Slughorn, it was poisoned. Fortunately, it was thwarted. Unfortunately, Slughorn gave it to Ronald Weasley. Harry Potter shoved a bezoar down his throat, saving him.