Honeycomb Yard no Dragmakhia

A mysterious catastrophe drove the humanity out of the lands.
Now surviving people live peacefully in an underground shelter called "Honeycombyard".
The lost world has been going away from people's memory across generations.
An assistant detective Haruto Kisaragi, who works at Ayaka Amagi's detective office,
is ordered by Ayaka to search for a stray cat Lieselotte with his coworkers.
When they find Lieselotte she transforms into a monster. Haruo also transforms
into a dragonewt, getting into a combat. A digital novel of supernatural ability battle mystery.

[From DLsite English]