Hei left syndicate & tried to be free, but heads of syndicate want him to be killed. After some time Hei meets Suou (young girl-contractor) and journey – that`s short season two description (you can check full description here)
In OVA series you can see time between first Darker than black season (I`ve released hd version too) & second season. OVA`s will be released on every even Blu-Ray volume.

[Grohotun.com] Darker Than Black 2 Complete Series (12EP + 4OVA) [BDrip 720p AC3 2ch EngSub]
If you have downloaded episodes before, U can use them in downloading complete release for speeding-up.
Also you can download episodes here:
[Grohotun.com] Darker Than Black 2 – 01 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 02 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 03 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 04 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 05 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 06 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 07 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 08 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 09 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 10 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 – 11 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 OVA 1 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 OVA 2 [BDrip 720p AC3 2ch EngSub]
[Grohotun.com] Darker Than Black 2 OVA 3 [BDrip 720p AC3 2ch EngSub] rev1
Darker than Black 2 MU
Darker than Black OVA`s MU