Thermal Blast is a powerful offensive ability in which the user can blast fiery heat energy from the hand. A Thermal Blast can be so powerful that it can knock out ten demons at a time. However, it isn't necessarily lethal as a mortal survived a direct hit with this power. This power is unique to Mercury Demons.
This power is presumably related to Thermokinesis, the ability to manipulate temperature. It is also similar to thermal balls, a power which is exclusive to Sirk.