The Soul Eater is the boss of the 3rd wave. It was defeated before Naofumi and the other heroes arrived. The Soul Eater is responsible for unlocking the Soul Eater Shield which initially contained the equipment bonuses 「Second Shield」, Soul Resistance (Intermediate), Mind Attack Resistance (Intermediate), SP Up, and Sp recovery (Small) as well as the special effect 「Soul Devour」. Found in chapter 『Chapter 82』 of the web novel.
The other world seems to have domesticated Soul Eaters and they were used to destroy the souls of Tact and his allies to prevent them from reincarnating.