Thantos' demon form is his ultimate form using dark energy (thantos' source of power), he uses deadly attacks and combonations. When this demon form is combined with mori, the Sacred Sword of Darkness he becomes nearly unstoppable.
One of the five sacred swords forged by the elves over 500,000 years ago. Mori was forged by the Drow's (dark elves). Thantos can draw limitless amounts of dark energy from it and on top of that it is capable of nuetralizing all light attacks that it blocks which is thantos' onyl real weakness.