special staff
Refers to a special staff officer or a special staff group, typically in military or government contexts. Not an everyday word; mostly encountered in organizational or historical descriptions.
He was appointed to the special staff at headquarters.
幕僚 is a general term for staff officers or aides, while 特別幕僚 specifies a special or extraordinary staff role.
Compound of 特別 (special) and 幕僚 (staff officer/aide). The exact historical derivation is uncertain, but the term follows standard Sino-Japanese word formation.