The Killing Curse is the last of the three Unforgivable Curses. Professor Moody tells his fourth-year Defence Against the Dark Arts class that there is no counter-curse to the spell and no blocking it. Only one known person has ever survived the spell: Harry Potter. When Moody demonstrates the Curse using a spider, there is a flash of blinding green light, and a rushing sound, as though a vast, invisible something is soaring through the air. The Curse leaves no marks or blemishes on the victim.
The Killing Curse needs powerful magic behind it to work properly. Moody says that if the fourth-year students were to attempt the Curse on him right then, he doubts he would get more than a nose-bleed.
Voldemort uses the Killing Curse against Harry. The jet of green light that comes from Voldemort's wand meets the jet of red light created by Harry's retaliating Disarming Charm in mid-air, causing a strange effect: a beam of bright, gold light connecting the two wands.