Night soil is a euphemism for human feces, which is excreted into a container or bucket, and is sometimes collected in the container with urine and other waste. In urban areas, usually slums, a night soil collector will arrive regularly, at varying time periods depending on the supply and demand for night soil collection. Usually this occurs during the night, giving the night soil its name.
Excrement is a more general term for feces, shit, and crap... but you know that already. I wonder what China/Taiwan editions of Granado Espada call this item? 魔兽屎? 妖大便? Heh.