Origin

The Pleiades, also known as the Seven Sisters, is an open star cluster located in the constellation Taurus. It is one of the closest star clusters to Earth and is visible to the naked eye. The Pleiades cluster is composed of hot, young stars formed from the same molecular cloud approximately 100 million years ago. In various cultures worldwide, the Pleiades holds cultural and mythological significance and is often associated with stories, legends, and celestial navigation.

On Pixiv

Most of the works in Pixiv that's tagged with this are actually the group names that come from the series name "Overlord." Pleiades refers to a group of powerful, elite battle maids loyal to the Tomb of Nazarick.