If you play through all of your demons, and don't get any extras from Kabal Courier or Kazakus, there will be 12 Demons in the graveyard by the time you play Bloodreaver Gul'dan. 4 of those will essentially be Voidwalkers. There's also a 2/4 with taunt and a 4/4.
Assuming The Darkness is taking up a board slot. You will resummon at most 6 demons. I understand the appeal of Voidlord, but is it almost counter-synergy with your DK because it dilutes the graveyard? Resummoning Voidlord would obviously be great, but for any individual board slot, you are 4x as likely to resummon a Voidwalker.