A rare document, the diary of a slave raider, offers a unique view into the sociopolitical situation at the turn of the nineteenth century in the colonial backwater of North Cameroon. The Fulbe chief in question, Hamman Yaji, not only kept a diary, but was by far the most notorious slave raider of the Mandara Mountains. This article supplements the data from his diary with oral histories and archival sources to follow the dynamics of the intense slave raiding he engaged in. This frenzy of slaving occurred in a ‘colonial interstice’ characterized by competition between three colonial powers – the British, the Germans and the French, resilient governing structures in a region poorly controlled by colonial powers, and the unclear boundaries of the Mandara Mountains. The dynamics of military technology and the economics of this ‘uncommon market’ in slaves form additional factors in this episode in the history of slavery in Africa. These factors account for the general situation of insecurity due to slave raiding in the area, to which Hamman Yaji was an exceptionally atrocious contributor. In the end a religious movement, Mahdism, stimulated the consolidation of colonial power, ending Yaji's regime, which in all its brutality provides surprising insight in the early colonial situation in this border region between Nigeria and Cameroon.