Sanders of the River by Edgar Wallace

There is a people who live at Ochori in the big African forest on the Ikeli River, who are called in the native tongue "The Keepers of the Stone." There is a legend that years and years ago, calacala, there was a strange, flat stone, "inscribed with the marks of the devils" (so the grave native story-teller puts it), which was greatly worshipped and prized, partly for its magic powers, and partly because of the two ghosts who guarded it.

It was a fetish of peculiar value to the mild people who lived in the big forest, but the Akasava, who are neither mild nor reverential, and being, moreover, in need of gods, swooped down upon the Ochori one red morning and came away with this wonderful stone and other movables. Presumably, the "ghosts of brass" went also. It was a great business, securing the stone, for it was set in a grey slab in the solid rock, and many spear-heads were broken before it could be wrenched from its place. But in the end it was taken away, and for several years it was the boast of the Akasava that they derived much benefit from this sacred possession. Then of a sudden the stone disappeared, and with it all the good fortune of its owners. For the vanishing of the stone coincided with the arrival of British rule, and it was a bad thing for the Akasava.