Once you have the key, go to the very center of the Temple of Skaen (M31,6), open the door and approach the Blood Pool. While talking to the pool, you will be able to sacrifice one of the party members. As a reward, you will receive the Effigy's Resentment talent: The name of the party member, e.g. Effigy's Resentment: Kana. This Talent provides a permanent bonus to statistics, but the exact characteristics depend on the party member that you sacrifice. Usually, it is +1 to one of the character's statistics and several points to one of the defense statistics. This is not a price that would make the sacrifice worthwhile. Above all, do not sacrifice one of the companions that you have encountered on your way. In an extreme situation, you can sacrifice a companion that you have accepted at the inn.