My thoughts so far are as followes
1.Fel Guard is just horrible. He is a 3 mana cost one less than a tazdingo for way too harsh of a penalty if you ever play this card in any situation other than on turn 10 it is not viable and will put you horribly behind your opponent. Either change his stats or the shatter 1 mana crystal.
2.Pit lord. He is a very beefy card to play on turn 4 or even later, but he is also a big target for any kind of removal it gets sheeped or hex'd and you basically just did 5 dmg to yourself for nothing if you ever play this card you better have a demon sacrifice ready to heal yourself as soon as you play it. And right there your already down 2 cards. Why.
3.Void terror this card is the same as pitlord it can be combo'd with power overwhelming to give you a super beast, but again giant magnet for any removal.
Now my opinons, any of these cards could use a change for the better ever other card imo the warlock has is okay decent or amazing. Twisting nether is a 8 mana board clear on both sides which is ment to kill off big scary things your opponent might of played so imo Its a okay card.
Their is one demon they have left out of the warlocks summon's where is the cute little doggy? Why did you add void terror and not the spell breaking hound? I would totally pick up that card considering it would definitely have a battle cry silence. That and maybe a card like the warlocks talents where if you have demons on the field and you cast the spell they are upgraded to their improved forms as in wow a succubus becomes the lady with 6 arms.
Again this is a changes / opinons topic and Id like to see a lot of reply's to what other people have in mind.