User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-60.53.97.0-20130601120329/@comment-33447191-20130601222745

Basically, it summarizes the featuring short stories that are introduced in Volume 15:

1. "Mahou-shoujo Ria☆Seriously!?"

2. Short Story of the fated meeting between Akeno and Rias.

3. "Holy☆Maiden goes to the Holy land!"

4. "Let's go for a training!" [Hell side]

5. "Wolf's Emblem" (Vali story 2)

6. Finally, The Titular Short Story "Dark Knight of Sunshine" (Kiba's Past)

Also the canon material of the Volume discussing about

'''"Hey, hey, Sekiryuutei-chin. Can I suggest you one things" "What" Kuroka points at Le Fay. "Can you have Le Fay as one of your candidate as a magician you will make a pact with?" "--!" Everyone here gets shocked at Kuroka's unexpected suggestion!'''

It ends with the introduction of Gasper and his life in Volume 16 as well as Rias' Special Move that she was developing in Volume 14.