I am currently playing a Warlock-Daggerspell-Shadowdancer drow and it is a very funny build. When it get HiPS it slays very hard with Hideous Blow (
which has been changed in this server and it is not needed to be recasted every round). I first took thief for the sneak attack requirement for Daggerspell, but recently RCRed it and took 3 lvls of Bodyguard (aka Neverwinter 9). This works very good, although its main problem is that HiPS must be taken later.
One recommendation I'd give you is, whatever the final build you decide to make, go for a dexterity build and drop strength and charisma. This means you are not going for a DC warlock, but yes for a damage-damage-damage. Let me explain it. As a Warlock/Daggerspell as main classes, you will be suffering for a medium AC (even buffed), maybe not in the firsts levels, but wait until you lvl up. The dex will aid your AC a bit and your AB if you take Weapon Finesse. When in a party, you can always blast mobs as a common warlock and, as you will be using two daggers (the main restriction this build has), their base damage will be added to your blasts. Dex also makes a great sinergy with sneakers, so here is where HiPS enters: is both defensive and offensive (in fact, is one of the best feats of the game). And going out and into the shadows while you backstab things is like
muahahaha 
.
Now seriously, this being said, I do not know if you like or have tried sneak builds, but this is a versatile and funnycombination. I did not play any sneaker until I tried this build, but I am enjoying it a lot. And the most important fact: you can give it a lot of interesting RP backgrounds.
I hope this can be of use to you. Feel free to ask if you have further questions.
