koro-sensei

The central character of Ansatsu Kyoushitsu.

Koro-sensei is an octopus-like creature with an ability to move at an incredible speed of Mach 20 and has managed to destroy 70% of the moon and threatened to do the same on earth in one year later had they cannot find someone who is capable of killing him. During this time, Koro-sensei was given a job as a teacher to teach various underachiver students in a government-sanctioned class that attempt to find a way to kill him.