It was during the reign of Nzinga Nkuwu that the colonists arrived from Sao Thome, setting in motion a pattern of cooperation and exploitation.  
Much of its power was exercised by the "royal guard" which was recruited among the Hum and Tio peoples.  Its ruler was elected by special body of nine or twelve nobles, including the pagan kbunga, who retained a veto vote, but rarely exercised it.  At one point this empire, whose currency consisted of a particular type of seashell named the nzimbu, was led by a woman named Anna, who was initially forced to retreat to the neighboring Kingdom of Matamba, before rallying her Jaga warriors to subjugate Luanda.  It was during the reign of Nzinga Nkuwu that the colonists arrived from Sao Thome, setting in motion a pattern of cooperation and exploitation.  Led by rulers that bore the honorific Mani, for many years its capital was located at Sao Salvador.  For 10 points, identify this West African kingdom that was conquered by the Portuguese and existed south of a namesake river.
Much of its power was exercised by the "royal guard" which was recruited among the Hum and Tio peoples.  
Kongo empire