Katsuobushi (aka bonito) is dried, fermented and smoked tuna and it's incredibly hard. It's so hard that it's possible to fashion a shiv out of it.
To do so, you'll need a mandoline, an adjustable wrench, a metal file, a vise to hold it in, an oven, a whetstone and some patience. YouTuber kiwami japan shows the way.
You'll not only get a dangerous weapon out of the deal but also a big bag of bonito flakes (which are great for making your food look like it's moving).