Thank goodness for the link in the Pixiv description, or I might never have figured out the logic behind the title. As it is, I'm still not sure I translated it adequately.
Wait, how Renko and Merry knows Rinnosuke exactly..?
Serious Scares: Sealing-Rin
The first half is based on the title of this site, which collects "stories so scary that it's no joke" (share ni naranai kurai kowai hanashi). The second combines Rinnosuke with "The (Secret) Sealing Club", the two-member club formed by Maribel and Renko.