Ryūjin

was a demon who commanded Tōshū the human swordsmith to create a sword for him and was later killed by him, following the blade's completion.

History
He first appeared in manga chapter 386, where he killed a couple of villagers while searching for Tōshū but then was quickly chased off by Inuyasha. Tōshū claimed that he met Ryūjin a year ago while travelling through the remains of a battlefield, and that Ryūjin demanded that Tōshū forge a sword for him using his scales. It was later revealed that it was, in fact, Tōshū who'd offered to make the sword for Ryūjin, as Tōshū wanted to use Ryūjin's scales to make his sword the strongest in the world. To ensure that Tōshū complied with his demands, Ryūjin marked Toshu's eye.

He appeared again after Inuyasha's group decided to protect Tōshū, and managed to repel Inuyasha's Wind Scar attack with his shield. His success lay in the fact that his shield was also made out of his scales, thus enabling it with the power to absorb youkai energy. His weakness, however, was that parts of his body lacked scales, as he'd ripped them out to forge his weapons. Inuyasha breaks his shield and injured him heavily, making it easy for Tōshū to kill him.