A Borg vessel with a complement of 11,000 drones, one of which was separated from the Hive mind in early 2377, thanks to a virus introduced into the Collective by the U.S.S. Voyager.

Sphere 878 was patrolling Spatial Grid 091, when the Borg Queen noticed that that single "voice" had become silent, so she initiated the sphere's self-destruct mechanism, thus silencing all 11,000 voices.

The Queen's motive for destroying the ship was in effort to convince Captain Janeway that her attempts to undermine the Collective would lead to thousands -- or millions -- of deaths.