The savior is revealed to be Carla Grunwald, a psychic from Ravenswood. ("Now You See Me, Now You Don't") According to Carla, she foresaw Alison being buried alive and went looking for her. She found Alison just as she was trying to pull herself out from the ground.
A DAngerous GAme" Alison's hand comes out of the ground similar to what happened in "This Is A Dark Ride". More of what happened during that flashback was revealed in the Season 3 finale when someone pulls her out of the ground, but only the Savior's arm and hand are shown. Because of this savior, Alison survived the vicious attack.
The Liars are in Ravenswood, Carla Grunwald approaches them and tells them that she went to go see Alison that night, after having a horrible premonition about her. She went in her backyard and saw Alison's hand poking from out of the ground, reaching for dear life. She pulled her out and said that Alison was confused, and her head was bleeding. Carla drove her to the hospital and left Alison in the car to get help. When she came back, Alison was gone.