A lawgiver
Lawgivers enforced the will of Landru when other measures, such as absorption, failed. Dressed in drab robes, they carried staves that appeared to be hollow tubes -- but that were capable of gathering and using some form of broadcast power. The staves could kill, and they could perform some form of the absorption process. Lawgivers existed in sufficient quantity that several could respond to any particular incident, and they were easily contacted by members of the population who needed them.
Kirk's experiences suggested that the lawgivers were under extremely deep control -- far deeper even than the control exercised over most of the populace. They were essentially incapable of independent thought, taking all of their direction and volition from Landru. Because of this, they were initially unable to handle disobedience. Their society was based on absolute obedience to Landru or his representatives, and the breakdown of that left them confused. Landru quickly corrected this flaw; when subsequently presented with disobedience, they threatened death immediately.
During Kirk's discussion with Landru, the machine was forced to use all of its reasoning power to debate him; this caused several lawgivers to enter the Hall of Audiences in a panicked state, begging for guidance. (TOS: "Return of the Archons")