havent seen many great demon decks around so i tought ill try making one ;)
biggest problem is getting them on the field with the high mana cost, thats wy i use solemn and dark ritual to get faster to mana and underworld connections and bloodgift demon for faster card draw, and quicksilver amulet to put them cheaper on the field. and some deathtouchers to keep me alive for long enough to get the big ones out
most cards i used are ones i already have (no i wasnt crazy enough to buy a griselbrand, but a friend did and i use it now :D )

http://deckbox.org/sets/308938

you should try putting elbrus, the binding blade in your deck to give some creatures a boost and have a big threat to the opponent. :D
although it dose cost alot of mana... :(

If you're going mono-black legacy you're going to need Dark Rituals. Include Crypt Ghast from GTC and you could mitigate the high casting costs of most demons. I'd also look into things like Demonic Taskmaster and Skirsdag High Priest - sack Reassembling Skeleton, guaranteeing a morbid trigger, and then you can produce a 5/5 flying demon token every turn.