Meiōjū

was a demon that possessed an shell impervious even to Inuyasha's Kongōsōha. Meiōjū was absorbed by Mōryōmaru, who desired to use the demon's strong shell as a defense against his enemies, especially Naraku.

The only attack known to have been able to breach this shell was Sesshōmaru's Dragon Strike, though his sword Tokijin broke as a result. Meiōjū had a powerful attack called Raimeihō (雷冥砲), which were spheres of black lightning.

Coincidentally, his seiyū, Tadahisa Saizen, also voiced Gōra, who is also a turtle demon.

Manga vs Anime
In the manga, Meiōjū, after being brought back to life by a piece of Mōryōmaru's flesh, assaulted a village in which his impenetrable armor was kept. After several attempts, and with the help of Byakuya, Meiōjū managed to reabsorb his armor, and was hit by Inuyasha's Kongōsōha. However, under the control of Mōryōmaru, Meiōjū was devoured along with his shell and the power of the Kongōsōha as part of Mōryōmaru's plan.

In the anime, Meiōjū was simply just revived and regenerated, only to be immediately devoured by Mōryōmaru in a quick scene. Furthermore, in the manga, Mōryōmaru was shown to be able to use Meiōjū's Raimeihō attack against Sesshōmaru and Kōga after devouring him. However, in the anime, this technique was never seen to be used by Meiōjū because he never fights.

Meioju