she was sacrificed as part of a ritual. Her corpse was chained to the ground over a magic symbol in the sealed-off forest, so either the long-dead summoner used her as part of his automatic summoning spell, or more likely as a way to make trees and nature grow in the wasteland (since she's an elf and all). Given the proximity to her corpse and the magic circle, the pillar likely absorbed her soul when he gave it life.