Hellish Holiday

The tour guide isn't technically a demon, but the places he takes you are painful and the sights horrific...

This is a day 1 deck, so I can't really say how strong it is. It does feel like there are some powerful things in here.

Mulligan

Keep the Soul Fragment cards, in particular Spirit Jailer. Imprisoned Scrap Imp is also good when drawn early. If you already found one of those, consider keeping Felosophy.

Against decks that like to buff their minions, you can keep Brittlebone Destroyer.

The deck has a low curve, so don't be afraid to toss everything that's OK but not great, since you'll likely get something playable back.

Game plan

Cheat out a lot of stats using buffs from Scrap Imp, free minions from Diseased Vulture, discounted Flesh Giants, buffed Void Drinkers.

Synergies

Expired Merchant can discard Hand of Gul'dan to draw you key cards like Flesh Giant and Void Drinker, but if you already have those in hand, it gives you more of them. Drawing cards later in the game can be useful to find healing.

There aren't a lot of demons in the deck, but the specific ones you can copy with Felosophy are very useful. I wouldn't recommend copying Flame Imp unless you're playing against a very slow deck. Make sure you check what it will copy before you cast: I've seen Felosophy glow yellow when it was in an outcast position, even though I didn't have any demons in hand.

If you have Diseased Vulture or Brittlebone Destroyer to play on turn 4, it can be worth using Tour Guide on turn 3 to set up the 0-mana hero power.