This is an African spice little used outside of the continent, so I've never cooked with it.
Negro pepper plants
Negro pepper ripening
Dried negro peppers
Negro pepper is native to tropical Africa (Ethiopia to Ghana), where the related species x. aethiopica and x. striata are both used for local cooking. In South America, x. aromatica (burro pepper), has found similar applications among Brazilian Indians.
Fruits. These are small, dark brown, cylindrical, twisted bean-pods with the contours of the seeds visible from outside. Each pod contains 5 to 8 kidney-shaped seeds grains approximately 5mm in length. The hull is aromatic, but not the grain itself.
Annonaceae (custard apple family).
For the etymology of "pepper" and "negro", see pepper. The genus name xylopia is a compression from Greek xylon pikron "bitter wood". Species name aethiopica refers to the origin of the tree in Ethiopia, although the plant now mostly grows in Ghana. In Cameroonian cookbooks the spice is referred to as kieng, the origin of which is unknown.
The diversity of tropical African growing countries is reflected in many names for the plant, e.g. Arabic fulful as-Sudan, English "Senegal pepper", French piment noir de Guinée, Portuguese pimenta-do-Congo and Russian Mavritanskij perets. Various references to Selim (e.g. English "African grains of Selim") are a reference to Moorish traders of the spice, Selim being the name of two Ottoman sultans.
Negro pepper was once used as a pepper substitute in Europe, but with regular imports of black pepper from India starting in the 16th century it largely disappeared. In later times, negro pepper was only traded as a pepper substitute in times of war and short supply. Nowadays it is rarely available outside of the production countries.
The term "spice" is associated with hotness and pungency although only few plants are able transmit a pungent quality to food. The fact that chillies were not available in Europe, Asia and Africa until the 16th century explains the historical importance of negro pepper and its substitutes.
It is not altogether clear why humans like pungent food and there have been numerous different explanations, some of which have become urban myths. Three examples are (a) Indian curries are supposedly a device to mask the flavour of unfresh ingredients, (b) the high price of pepper made it into a symbol of wealth, (c) culinary use of pepper arose from its previous medical use. It is more probable that there are sound biochemical reasons. The body interprets hot flavour as pain and reacts by secreting pain-killing endorphins, which have analgesic functions and also show stimulating and euphoric power.