|Skeleton found with death whistle at Tlatelolco, part of the Aztec Triple Alliance.|
These things are amazing. I want one. I want everyone I know to have one. Watch the video and use it to scare your family at dinner today. The screaming starts 50 seconds in.
It would be pretty simple in game terms. Every non ally in the vicinity is under the effects of a fear spell (save or flee in terror). Most would have been made from fired clay, but they could conceivably be carved from obsidian or jade if you want to use them as treasure.
Jose Luís Franco, a Mexican engineer, published the first academic drawings of the whistles in 1971, pictured above. The one on the right is shaped like an owl because it symbolized death and destruction to the Aztecs. Apparently they weren't an original invention of the Aztec culture, and were used by the Olmecs as far back as the 8th century BCE.
Both of those images came from this article which is pretty informative.