Obligatory

[Sarah]

Binding in law or conscience; imposing duty or obligation;requiring performance or forbearance of some act; -- often followedby on or upon; as, obedience is obligatory on a soldier.As long as the law is obligatory, so long our obedience is due. Jer.Taylor.

[Sarah]

ADJ. Morally or legally constraining and/or binding.
ADJ. Required by obligation.

[Lanell]

If something is obligatory, you must do it because of a rule or law, etc.:

[Jessica]

Required by a legal, moral, or other rule; compulsory.