Wasabi (made of a root that grows in clear mountain streams in Japan - a rare delicacy - most probably not what you usually get served under that name here) is not mustard which is made out of mustard seeds. The taste is different as is the origin.
If only the fierceness is of concern, than there is nothing like the breeding of an American guy who is hunting the world record for the hottest chilly with his Carolina Reaper varieties. I think his last breed was near to 2.7 Mio. Scoville. There is a natural limit as to how hot it can get from any kind of food at 8 Mio. Scoville, which is pure Capsaicin (and which would probably severely damage your health if not kill you)
