The Ruins of Tribulation (episode)

Teh Ruins of Tribulation is the fourth episode of Koro-Sensei Quest! 

Overvies
Can the Demon King be accepted as a teacher at Kunugigaoka Magic Academy for the Class E? Korosensei is put to the test by national hero Pope Gakuho Asano himself.

Summary
The Class E and Korosensei are summoned by Gakuho Asano to a myterious dungeon, were they meat a Kunudon who makes Korosensei and Class E various obstacles and at the end the final obstacles which is to either sacrafice himself for teh students or press a button and save himself by being crushed, Korosensei pressed the botton and saved both himself and the students.