The Royal Authority of the Moon. It would normally appear as a crown, but for the sake of being easy to equip, it is in the shape of a ring.
It confers the Royal Authority to freely use the functions of the Moon Cell, sort of like administrator privileges. The Regalia is a system created by the Moon Cell to resist Velber. When the Umbral Star arrives, the Moon Cell enters a state of hibernation, completely shutting out all invaders. It subsequently leaves the security of its inner areas to an Al or Servants it trusts, with the Regalia marking this.
The only ones who can destroy the completed Regalia are its owner and Sephyr, the Anti-Cell. However, if some accident should cause it to split into multiple pieces, destroying it becomes a simple task.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA