Avada Kedavra kills things, that much is known, but what kind of damage does it actually do? Does it just stop all bodily functions? Does it force the soul to pass on? Damage the soul? Has J.K. Rowling actually explained the way it works in any way?
One thing does point to the bodily death only, as apparently Fawkes, an immortal phoenix, could swallow the spell and be reborn with no harm (apparently) to its soul.
Fawkes swooped down in front of Dumbledore, opened his beak wide, and swallowed the jet of green light whole. He burst into flame and fell to the floor, small, wrinkled, and flightless. […]
The hall was quite empty but for themselves, the sobbing Bellatrix still trapped under her statue, and the tiny baby Fawkes croaking feebly on the floor— […]
He did not look at Harry at first, but walked over to the perch be- side the door and withdrew, from an inside pocket of his robes, the tiny, ugly, featherless Fawkes, whom he placed gently on the tray of soft ashes beneath the golden post where the full-grown Fawkes usu- ally stood.
— Order of the Phoenix, chapters 36 and 37
But could that also mean if you can get the heart pumping again the person could be restored? Or is it more of a "irreparable magical damage to the body" thing?
(Do note this is asked mostly from the point of view of crossover battle speculation, i.e. would immortals of another verse with no magical resistance be impervious to Avada or not.)