A new promo video was streamed by Kadokawa about the OVA that will ship together with the 11th volume of Overlord on September 30. The new animation will contain 30 minutes of new footage of the Pure Pure Pleiades chibi anime short series, and will feature the appearance of the Pleiades as well as the rest of the Floor Guardians.
Video: https://www.youtube.com/watch?v=hfJ5PER_GH0
Source: http://www.animenewsnetwork.com/news/2016-07-29/overlord-ova-ains-promo-video-streamed/.104793
A new teaser was released by Kadokawa.
Video 2: https://www.youtube.com/watch?v=NSU6Rpb9b0A
A third video was released by Kadokawa.