Information and Structure of a Passive Periphrastic.

A passive verb form consisting of the GERUNDIVE, a common term for the future passive participle, along with a form of sum.

• The gerundive, as essentially a predicate adjective, agrees with the subject of sum in gender, number, and case.
• The gerundive often conveys an idea of necessary, obligatory, or appropriate action, rather than simple futurity, and this is the case in the passive periphrastic.

(nominative subject) + (dative of agent) + (future passive participle/gerundive) + (verb to be)

People ought to seek for peace.

Peace was to be sought by men.

The king is to be (must be) banished. [can't put this phrase into the active because there's no dative of agent]

The kings are to be (must be) banished. [can't put this phrase into the active because there's no dative of agent]