Hi, I have just started playing magic again after a 3 years brake, and i have created this monoblack deck. I have played with it a few times, and i won almost all of the matches, but i just fell that theres something there needs to be changed for perfecting it. So i hope i could get some suggestions.:)
I don't know what format you're trying to make this, but this is definitely not Extended legal. Extended only encompasses the last couple of blocks.
This would be Legacy Legal, except Demonic Tutor is banned in Legacy. Even in Vintage, Demonic Tutor is restricted to one copy per deck. If this is purely casual, that might be okay, just make sure that the people you play with are okay with you having a deck that runs multiple Legacy Banned/Vintage Restricted cards.
Also, this mostly looks like an aggro deck, I'd suggest dropping the Sorin's Vengeance, Damnation, Kokushos, and Animate Deads. That frees you up five slots.
If you're going to run Abyssal Persecutor, you want more ways to get rid of it yourself or you're going to have trouble closing out a game, as currently only your 2x Cabal Therapy can do this. Try swapping your Terrors for Go For the Throats, which trade the mostly-irrelevant "can't be regenerated" clause for the ability to hit black creatures including your Persecutors. With your additional slots from before, you can bump up Cabal Therapy to a 4-of, and consider adding Innocent Blood, Gatekeeper of Malakir, or something of the like that adds more removal and can also be used on your own Persecutor.
Also, Black Knight seems weak. Consider Dark Confidant, or if the price tag on that is too much of a limit, Nantuko Shade
Like I said, everyone's definition of "casual" seems to be a little different. I'm of the opinion that "anything goes" isn't really casual, because you'll sit down and find yourself against a deck with 4x Sol Ring, 4x Strip Mine, 4x Demonic Tutor, and other ridiculously overpowered stuff that even in the most open of formats is limited to 1 copy per deck.
The Shade is worlds better than the Knight. Neither first strike nor pro white are too terribly relevant, and the shade gives you things to do with your later in the game, when you're in topdeck mode.