(Final release is scheduled for 24th May, 2017 )
The story follows Kojou Akatsuki as he takes on monsters and organizations(evil or not). During this journey he gains more control over his familiars and his own vampiric powers. Technically STB II OVA's are a sequel and thus continue from where the main series left off. That being said it is also important to mention that the main series had 24 episodes (plus 2 OVA) with a somewhat conclusive ending.
So if you want you can watch the OVA as a standalone. The downside to this approach would be that the characters would seem to pop
randomly in and out without making any sense but the story of the OVA itself would not suffer much. STB II has eight episodes that seem to be divided into mini arcs of 2-3 episodes each.
First three episodes - sword shaman of shadows
Next two episodes - The 4th Primogenitor on the run
Last three episodes – Knight of the Sinful God
As you can guess that small a run time obviously leaves much to be desired. If you are a fan of the series then you would be left clamoring for more. For people who are new, I seriously recommend that you watch the first 24+2 episodes of the main series. Not only is first series a great watch but it would probably make the watching of STBII much more enjoyable. If you choose to watch STBII as a standalone then don't expect much in terms of good character development. The story itself might interest you enough to keep your attention but it definitely wouldn't do justice to the series itself.
Major big reveals are made about a lot of our MC’s in this second season. First season was really good and like all good series it held back on lot things about the main characters. So now it was only obvious that the second season should answer some of those mysteries. If you have already gone through the source material then obviously nothing much is going to surprise you but if you are just watching the anime (like me) then it’s satisfying to see a few if those mysterious questions getting answered