Resshin

Resshin is the eighth Flame Dragon and is actually Recca's father Ōka. His flame can engulf his opponent and either turn their soul as part of Recca's flame or completely disintegrate the soul. This is Recca's strongest move. He uses this on Yanagi when she dies and makes her apart of his flame. 

Source: http://flameofrecca.wikia.com/wiki/Resshin