Culinary equivalent of horseraddish, an explosively spicy Japanese condiment.
It's also the cure for infection by the Medusoid Mycellium (deadly mushrooms), didn't you know? You can find a plethora of the Mycellium in the Gorgonian Grotto (aka the Grim Grotto) if you're like Count Olaf, hoping to use them as a volitile weapon to attempt to conquer all that is good and noble. Beware, they wax and wane by the hour.
Count Olaf: Hahaha you orphan brats, I've poisoned your stupid chef-baby with the Medusoid Mycellium! Now it will die and I will soon get my icky poo poo hands on the Baudelaire fortune!!! (exeunt)
Klaus and Violet: We have no horseraddish to save Sunny! Oh no!
Sunny: *coff* *cofff* Wasabi *coff*
Klaus and Violet: Of course! Wasabi!