The Prime Lord began to reveal his full power after he realized the real danger in the situation he was in. He thought that those three wouldn't be a problem at all until he found out that his life was in danger. He used hellfire to coat his body, which could easily melt steel by just being near it as a defense mechanism.
The problem began to appear when Uriel was unaffected by the hellfire. Her black sword and even the robe she wore didn't melt by it because of her Aura. She ignored the black flame that acted like a shroud that could move freely by the Prime Lord's will. She could cut through the hellfire and his body like it was nothing.
