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You have a lot of stuff that doesn't mix in competitive games. You have a good start, but I would turn the paladin squad into a terminator squad and have two. You will need another dread knight, they run much better in twos. They are the heavy lifters of the army. I would also make draigo a librarian or something. Purgation squads are just underwhelming.
Using only the models you have... use draigo as a librarian. use paladins as terminators. Combine strike squad and purgation squad and use as Purifiers (in a Rhino if you have one).
Other than that, you need more models. Another Dreadknight for sure.
Kabal Of The Flayed Skull 1500pts
Vior'la hunter cadre 2000pts
Brotherhood 2500pts
Daemoic Khorne Daemonkin war band 1000pts 
BLACK LEGION 
6000+
2500
2000
2000