Why the name "Deathfire" ? It's a mix of "Death" from Deathrattle and "Fire" from the Fire-related Elementals this deck is based on. So just refer to that archetype as "Deathfire", mainly because it sounds so awesome, but also for my ego. (XIII™)

What is The Caverns Below's activator ?

The Flame Elemental, given by Fire Fly and Igneous Elemental.

What are the ways to activate it faster ?

Useful cards to help completing The Caverns Below faster are Shadowstep and Shadowcaster, allowing to bounce the Flame Elemental. With luck, Bloodthistle Toxin also has that effect.

Also with luck, Mimic Pod and Barnes can give other copies of Fire Fly or Igneous Elemental. Spiritsinger Umbra with the latter also give 2 Flame Elemental in hand for the quest.

There are Bloodmage Thalnos, Loot Hoarder, and sometimes Kingsblood Toxin that can help drawing the Fire Fly / Igneous Elemental and combo options faster.

What to do after The Caverns Below is completed ?

When the Crystal Core is played, some Flame Elemental should be on the board which all become 5/5 due to the quest. Fire Fly thus helps to swarm, literally giving two 5/5.

There is in this deck a lot of cards with low stats compared to their mana cost, which therefore benefit a lot from the Crystal Core : Bloodmage Thalnos, Loot Hoarder, Igneous Elemental, Xaril, Poisoned Mind, Barnes, Spiritsinger Umbra, Shadowcaster, Vilespine Slayer.

Among them, Barnes and Shadowcaster are especially cool because their effect will provide 5/5 cards instead of 1/1 ones. They are perfect in that archetype, after or before the Crystal Core.

A forgotten card is here for the main purpose of abusing of the Crystal Core once completed : Sergeant Sally. It allows a crazy 5 damages board clear and the combo with Spiritsinger Umbra shouldn't be underestimated.

In late game, N'Zoth, the Corruptor acts as the game finisher. There are 7 Deathrattles minions : Bloodmage Thalnos, Loot Hoarder, Igneous Elemental, Sergeant Sally, Xaril, Poisoned Mind. It's possible to get more Deathrattles with Barnes and Shadowcaster. Even after that nice board is cleared, the Deathrattles activate and other options become available, including the possibility to fill the board with a new wave of Flame Elemental thanks to the Igneous Elemental.

What are the other good cards ?

Backstab : Quick, grants an early removal and allows an easily activation of the Combo effect of Eviscerate or Vilespine Slayer. Necessary against aggressive decks, and just good overall.

Preparation : Speeds up the deck a lot, it's also a combo effect enabler and it makes spells like Eviscerate, Shadow Strike or Mimic Pod to cost 0. In addition, it makes Crystal Core cost only 2, which means more minions can be summoned at the same time you play it.

Eviscerate : A necessary removal that can be used to burst and finish a game. The Combo effect can easily be activated with Backstab, Preparation, Shadowstep, the Toxins or even a Fire Fly and Flame Elemental.

Shadow Strike : A great removal to handle the early game before the Crystal Core. It can also be noted that it can activate a 5/5 Sergeant Sally and that it's great against another Quest Rogue.

Mimic Pod : A lot of awesome incomes. Can be better than Arcane Intellect from the Mage class, or worse depending on the luck. As there is a lot of combos in the deck, that's a good card overall.

Tar Creeper : This deck needs to handle the early game against aggressive decks, which this card does well. It also has the purpose to activate the Tol'vir Stoneshaper. On Barnes or a Shadowcaster, it's still a 3/1 Taunt... or a 7/5 Taunt once the quest is completed !

Xaril, Poisoned Mind : It synergies well with Spiritsinger Umbra / N'Zoth, the Corruptor and is even good with Shadowstep or Shadowcaster. All Toxins are interesting : Bloodthistle Toxin to bounce Flame Elemental, Briarthorn Toxin with Sergeant Sally and to trade with all the tokens, Fadeleaf Toxin can be interesting to burst or to prepare a Shadowcaster, Firebloom Toxin is nice because this deck doesn't have that much removals, and finally the Kingsblood Toxin is good to draw the Elementals faster. An excellent card throughout the game.

Tol'vir Stoneshaper : As you have a lot of early game Elementals (Flame Elemental, Fire Fly, Igneous Elemental, Tar Creeper), it's just awesome on the curve and it's hard to remove.

Vilespine Slayer : Same as for Eviscerate, the Combo effect is easily activated. It acts as a huge minion removal to take the board in mid-game. Also good with Shadowstep and Shadowcaster.

Are there nice combos ? What are they ?

With Barnes : In the first place Sergeant Sally is a 1/1 so she is okay. The 6 other Deathrattles I mentionned above are also excellent with him. If the income is one of the Battlecry minions, Shadowstep and Shadowcaster can help to make it still worth it. Finally, if he summons Spiritsinger Umbra, the opponent will have to clear it or the next turn could be god-like.

With Spiritsinger Umbra : It's simple : more draw with Bloodmage Thalnos and Loot Hoarder, more Flame Elemental with Igneous Elemental, more Toxins with Xaril, Poisoned Mind, and a sick board clear with a 5/5 Sergeant Sally (plus two 5/5 on the board, guaranting the control of it). Plus, if it happens to be on the board when you play N'Zoth, the Corruptor... Well, I'm dreaming.

With Shadowcaster : On Fire Fly to have another Flame Elemental (or directly on the latter), on Igneous Elemental for the same reason, on Sergeant Sally to basically have a 1-mana cost one, on the token given by Barnes, and on Spiritsinger Umbra for quick combos... Later on, every card you copy is 5/5, making things like Tar Creeper or Tol'vir Stoneshaper worth it as well.

Some showcases : https://www.youtube.com/watch?v=vb5y7ASvNCo by kiwiinbacon https://www.youtube.com/watch?v=9wGwQHaLHbs by Merchant https://www.youtube.com/watch?v=yzintdpd8Qw by Khristophesaurus

THANKS FOR READING !

TLDR : Everything combos with everything, very coherant deck.