Orb of Domination

The Orb of Domination (支配の宝珠) is a magical item imbued with Kagali's dominating magic. One was used by Clayman who believed he could control Milim Nava with it; and another was used by Reiner, who attempted to control Rimuru Tempest.

Abilities
The Orb allows its user to inflict their desired target with the powerful mental domination magic of the Kagali, who was once known as the Curse Lord Kazalim. It's incredibly powerful, capable of putting even a lesser Demon Lord under the user's complete control.

While Clayman put Milim "under control" by having her wear the orb on her body, it seems that it's also possible to activate it from a distance as attempted by Reiner.

Appearance
The Orb has the appearance of a glass-like sphere slightly larger than a marble. When activated it glows a faint purple light.

The type of accessory it was put into to control Milim Nava is different depending on the version of the story. In the novel and anime, it was embedded into a pendant; while in the Manga it was embedded into an arm band instead.

Original Orb
The original Orb of Domination was created by Kagali and owned by Clayman, who had it given to him by Yuuki Kagurazaka at an unknown point in time. It was used on Milim Nava in the form of jewelry, which Clayman mistakenly believed to have put her under control.

The Orb remains in the possession of Milim, with its fate remaining unknown.

Second Orb
Though the exact circumstances behind the creation of the second Orb of Domination are never elaborated on, it was sold to Prince Elric of Inglassia, most likely by Five Elders. The Orb was given to Reiner, who under Prince Elric's orders disrupted a Meeting between Tempest and the Council of the West in an attempt to subdue Demon Lord Rimuru.

The Orb was destroyed by Rimuru Tempest in secret, who removed its magic without destroying the orb itself.