As part of the Kamachi Kazuma 10th Anniversary Celebrations, Kamachi wrote a short story that was released a prize in the Ichiban Kuji lottery (Announcement #5).
The short story is titled Toaru Hourou no Imagine Breaker or A Certain Wandering Imagine Breaker. For a while, we had no details about the contents of the short story. Luckily, Debi from Trigger Happy End made a blog post about it here. (Its all in Japanese though. Also, no pictures for the SS).
The story occurs in the first 10 days of September (again) putting it somewhere between OT6 and OT7 (or between the 2 anime seasons if that's how you remember it ). (My guess is that it occurs on September 6 or 7th considering there is no other time that this could occur).
In a nutshell:
1. Someone is setting up a magical attack against Academy City (again).
2. The thing that will become the key for this attack is Kamijou Touma's sleeping time.
3. After this, within a 5 hour time period, if Kamijou is not sleeping for at least 2 hours (ie awake for 2 hours or more) the magical attack will commence! (I'm guessing this repeats every 5 hours. Don't know how this works so don't ask. Also: Fukouda!)
4. Because of this, Kanzaki Kaori arrives to put Kamijou to sleep. (literally, not figuratively)
Other characters who appear: Index (obviously); Himegami Aisa (hooray); Misaka Mikoto (obviously) and Shirai Kuroko (seriously).
Comedy and potential jokes about sleeping in the same bed together are bound to occur.
As for the effect of the magical attack in question, I don't know if Debi is trolling me or not but:
The effect is the destruction of all male genitalia in Academy City. Described as the most fearsome magic spell in all of the Old and New Testaments.
But again, I think that's just Debi being a comedian. Crazy if it was true though.
Hoping that more details can come from a better translator (and that js06 will get to this).