Oni Gauntlet
Central to the first and third major chapters of the Onimusha franchise (and to a lesser extent, the fourth chapter) is what is known as the Oni Gauntlet. This is the gauntlet that was given to Samanosuke Akechi in Onimusha: Warlords and was created by the power of the council of the twelve Oni Gods. It absorbs souls (see below: Nature of Souls), gives power to Samanosuke's weapons and changes the color of the orb in its center to match with that of the corresponding weapon, allows him to achieve an Oni Awakening, and is capable of functioning with other Oni items such as the Oni Army Orb.
Yagyu Jubei's mystical parentage through Takajo enabled him to use Oni power without the need for the Gauntlet. This was also repeated with Soki, who was revealed to be the reincarnated Oni God of Darkness. However, while Samanosuke and Jacques both had Oni Gauntlets, only Samanosuke could become the Ultimate Onimusha. It is likely that Samanosuke was chosen by the twelve Oni gods enabling this, while Jacques was granted power by only one of them. Because of this, he was only able to transform into one form. Additionally, Jacques' gauntlet began with a red sheen, turning to gold as it gained more power.
In the third game, Samanosuke had been sent to modern Paris. Thus, his gauntlet had reverted back to its original bluish color, and all of his previous weapons (Raizan, Enryuu and Shippu) were lost since he was in a different timeline. However, he was able to restore his gauntlet to its true potential by reabsorbing more souls, regaining new weapons (namely Tenso, Kuga and Chigo). Later, he was able to fuse the power of his gauntlet with the gauntlet of his alternate deceased self, when he returned to a slightly alternate timeline. This gave him two gauntlets, thus allowing him to achieve his Ultimate Onimusha form.
After defeating Nobunaga, he absorbed the Genma Lord's souls. Unlike the souls of other Genma, these were a dark-hollow, green color that gave the gauntlet and its gem a dark red sheen. This symbolized that the gauntlet had sealed away the essence of the Genma Lord. This was also the reason he had to seal the gauntlet away. Years later, as the God of Darkness was reincarnated as Soki, Samanosuke lost most of his power as the Onimusha. Thus, the Oni Gauntlet lost all its potential, turning his hair grey within a short period of time after losing part of the Oni Mystic Power. The only power that remained in the Oni Gauntlet was the sole power of the Genma Lord retained from Nobunaga.
In the fourth game, Tenkai/Samanosuke utilized this power against the Genma Army and even achieved an Oni Awakening with it, before it was transferred to Soki. At that point, Nobunaga's power was unsealed in the final duel against the resurrected Fortinbras. This Oni Awakening was a result of the Genma Lord's power since that was the only power left in the gauntlet, explaining why he was not able to use the same moves or the same weapons that he did in the previous games.
Read more about this topic: Onimusha