Dark Debt

Warlock Spell (E)
Draw three cards. Add three Demonic Interest cards to your opponent's hand.

Demonic Interest

Warlock Spell
Deal 2 damage.

Neat, but Warlock isn't exactly a class in need of card draw.

Let's take it up one notch:

I think Surrender to Madness should be 9 mana so you can curve into the T10 Y'Shaarj.

Surrender to madness 10
Spell E
Replace your hand and deck with infinite wisps.

Wisps of Light (2)
Paladin Spell (E)
Battlecry: Change your Hero Power to "(0): Summon a 1/1 Wisp."

Wisp Hoarder 2
Neutral minion C
Deathrattle - Draw 2 Wisps
2/1
Flavor: "He's just holding them for a friend"

Isn't that just a better Haunted Creeper?

You could argue that spawning them immediately gives greater tempo, that the 1/2 body is better, or that being a beast is significant. You could also make the claim that you can't draw anything not in your deck, and that the card is actually near-worthless.

Too lazy to format cards properly for a game with no custom card support.

Lost Avatar of Rafaam

Warlock Minion (L)
2/2
Start of the Game: Shuffle three Curses of Rafaam into your opponent's deck.

Young Reno

Neutral Minion (L)
3/5
Battlecry: Remove all duplicates from your deck.

Brann the Elder

Neutral Minion (L)
4/6
Your minions with Battlecry or Deathrattle have both. (Deathrattle targets are chosen randomly).

Professor Elise

Neutral Minion (L)
7/7
Battlecry: Shuffle Elise's Last Journey into your deck.

Elise's Last Journey

Spell
Your hero power becomes "Discover an Old God."

Sir Finley the Brave

Neutral Minion (L)
1/3
Inspire: Summon Finley from your deck.

Undertaker Mazov

Priest Minion (L)
8/8
Taunt
Battlecry: Gain the Deathrattles of all friendly minions that died this game.

Close enough?