Hi man, welcome amongst the Heretics!
I'll go to the point: do you want a background based army or a competitive tournament army?
Since i don't know your preferences, i'll suggest you something n between
For a really story based stereothipical Black Legion, you could buy Abaddon.
Otherwise i suggest a Biker (stronger)/ teminator ( fluffier) sorcerer or lord
If you choose Abaddon you'll want to get Chosens and load them with fancy weapons ( i suggest plasmaguns and meltaguns)
If you got the biker lord, you could decide to give him a Chaos Mark to unlock some Cult Troops from the Elite section (for more competitiveness). Otherwise I'd just go with black marines, with squads divided in clear roles :
10 man, all close combat + pistol, 2 flamers, power claw, to hunt enemy infantry and hordes;
another 10 man 2 meltaguns, rhino w/combimelta, to hunt enemy vehicles and Monsters; another 10 with missile launcher and plasmagun, maybe in a rhino w/ havoclauncher, for long range support
If you have Abaddon or Terminator HQ, go for 4 terminators, maybe with a couple of combimelta/plasma
If you have money to spare, get a Land Raider to roll with them as a dedicated transport.
Otherwise I'll skip this section
An heldrake is a must. ALwais. Don't ask. Buy.
If you choosa a Biker HQ, you'll want to escort him with 5 bikers. Choose a role for them and arm them accordingly (ful melta, full flamer, i'm not a fan of the full plasma here)
Raptors are an adequate choice either, maybe less resilient.
An havoc squad is mandatory. Your take on the weaponry.
Anti infantry: autocannon or heavy bolters
Anti Tank: missile launchers/lascannons
Anti Monsters: lascannons
You could mix similar weapons to add flexibility. EG 2 missile launcher & 2 Autocannons, or 2 heavy bolters & 2 Autocannons (only against tyranid!
Vindicator (harder to use) /Predator (just effective from the beginning) tank and obliterators might be your second (but not less effective) choice
So let's say:
Burning Brand of Skalathrax
Sigils of Corruption
2 Flamer (BURN!! BUUUUURRRRRNNNN!!!!)
ccw + pistol
(weapons at your choice)
2 predator tanks
115 + 115
TOTAL 1500. Not great at anti air...but that would depend on your local meta. Mine is absent flyers.