"Owari no Seraph: Kyuuketsuki Shahar" is an OVA that would be best secribed as poor, boring and bland.
The action takes place right between the two seasons of the anime so as to possibly avoid disturbing the original plot line, because it includes what one would consider "filler" content (it's most likely not present in the manga as well).
The OVA introduces two new characters that are just generic one-dimensional personas meant to evoke sympathy from the audience and nothing more. Despite their purpose, said characters are not only shallow and dry, but also extremely forgetable.
The story is rushed due to the time restrictions,
but quite honestly there was nothing much to show anyway.
There wasn't anything redeeming to the production as well.
All in all, you can watch this if you want to see some of the characters from the original series (members of team Shinoa and Guren, as well as Ferid, Crowley and Mika) but if you want to view this for entertainment purposes, you'll be greatly disappointed.