One of Two
Alternate timeline
(covers information from an alternate timeline)
One of Two was a prominent Borg Sentinel who lived during the 24th century.
At some point before 2401, One of Two was killed by General Jean-Luc Picard, and his skull was later mounted on display in his trophy room at Château Picard. (PIC: "Penance")
This Borg drone was only mentioned in writing.