"Nuclear horseradish." An extremely spicy condiment of Japanese origin, made from the root of the wasabi plant, ranking just below uranium in terms of destructive potential. Allowing any more than twelve molecules of this pungent spice to touch your tongue will cause your head to explode. Well, it will feel like that anyway. When used correctly, good for cleaning out the sinuses and warding away unwelcome guests. Wasabi is available in paste or powder form.
Commonly used in sushi, but also in various other dishes including sashimi, or to flavor udon, soba, ect. Keep away from the eyes. Do not inhale. Do not attempt to feed to pets. If you exceed the recommended dosage (approx 1/4 teaspoon) seek medical help immediately, lest your intestines disintegrate.
Much of the 'wasabi' available in European nations and the Americas is total crap and isn't much spicier than pepper. You'll know it's the real stuff when you start breathing flames and your appendix explodes.