Breath of Fire IV is the fourth RPG in the Breath of Fire series. It was released for the PlayStation in Japan and North America in 2000 by Capcom. In 2001 it was released in Europe.
One thing that was never mentioned in the game but was explained in the official source book was that the Dragon Slayer Sword was originally a God whose summoning was botched up by Yuna. Like many other failed summonings where the summoned gods appear incomplete, only the god's head appeared. Yuna then studied the head thoroughly and decided to crush it to pieces and use it to forge a sword. The God's desire to find its body and its hatred are still within the sword, and it tries to absorb the abilities of any god like itself.
If you have the power to destroy the world...then you have to take responsibility for the world...don't you? - Illusion of Ryu
