Overlord: Oukoku no Kan-tachi - Drama CD-tsuki Tokusou-ban
English name: Overlord: The Maid Tea Party
Genres: Comedy
Type: ONA, Episodes: 1
Aired: Jan 28, 2014
Source: Light novel
Rating: PG-13 - Teens 13 or older
Synopsis:
A picture drama "trailer" released on Enterbrain's official YouTube channel, the publisher of the Overlord light novel.

It promotes the deluxe edition of the 6th light novel volume that people can purchase which has the full audio drama released on CD included. 

The drama is about the Pleiades gather together for a tea break. Here, they discuss their recent missions and their dedication to Ainz Ooal Gown.

(Source: Overlord Wiki)