|What Comes Suddenly and Never Leaves?|
|Romaji Title||Totsuzen Yattekite, Kaerukoto no Nai Mono wa?|
|English Title||What Comes Suddenly and Never Leaves?|
|Airdate||April 24, 2014|
|Opening||Soushou Innocence by Maaya Uchida|
|Ending||Across the Fate by Haruka Yoshimura as Kouko Kaminaga|
|Previous Episode||Episode 3|
|Next Episode||Episode 5|
What Comes Suddenly and Never Leaves? (突然やってきて、帰ることのないものは？; Totsuzen Yattekite, Kaerukoto no Nai Mono wa?) is the fourth episode of the Akuma no Riddle anime series and was released on April 24, 2014.
As the Myōjō Academy's 10th year's Black Class undergo mid-term exams, Tokaku Azuma and Haru Ichinose hear from Chitaru Namatame, who is often seen with Hitsugi Kirigaya, that she is not targeting Haru, as she joined the Black Class for a different reason, before later hearing an urban legend from Nio Hashiri about a hidden room in the library.
Upon returning to their room, Tokaku and Haru discover another advance warning, also discovering some bombs has been planted in their freezer. The culprit this time around is Kōko Kaminaga, an assassin specializing in explosives who was exiled from her organisation NPO after her teacher was killed in a failed bombing attempt, which Kaminaga herself was responsible for, as she had set the timing that the bomb would go off at too early, which killed her teacher, instead of their target.
Despite knowing the danger, Tokaku accompanies Haru to the library and manages to locate the hidden room from Nio's rumors. There, Tokaku narrowly manages to protect Haru from a hidden bomb and pursues Kōko, who wanted to use her wish to quit being an assassin, managing to disarm her and stop her attempt, leading her to be transferred out of the Black Class the next day.