The kingdom suffered a constant threat from certain predatory tribes.

Wild : (of an animal or plant) living or growing in the natural environment; not domesticated or cultivated.
Violent : using or involving physical force intended to hurt, damage, or kill someone or something.
Rebellious : showing a desire to resist authority, control, or convention.
Plundering : steal goods from (a place or person), typically using force and in a time of war or civil disorder.
Predatory : seeking to exploit others.

Synonym of Predatory is Plundering