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 ?
What are the ways to activate it faster ?
What to do after The Caverns Below is completed ?
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.
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 ?
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.
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.
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.
Side cards that can enter the deck :
Moroes, good income on Barnes, awesome with Crystal Core because this is a 5/5 Stealth (and thus is very hard to kill) and he will have a lot of value, giving another 5/5 every turn. Though, it's only good when you have The Caverns Below completed so it may be slow.
Gluttonous Ooze is great against Pirate Warrior, especially with cards like Shadowcaster or Shadowstep to really annoy them. It also works against Wicked Knife from the Rogue's Dagger Mastery or Blood Fury from Lord Jaraxxus, and some Hunter / Paladin / Shaman also play weapons so it's not a dead card in most match-ups. Since it's early game-oriented, you can cut N'Zoth, the Corruptor for it.
Golakka Crawler can be considered if there are too much pirates in the metagame, which is the case at the moment (Pirate Warriors, Quest Rogue, Aggro Druid...). So, if these are the match-ups you want to target, they can enter the deck. It's a great card to take the board against these decks.
Fan of Knives is interesting against aggressive decks such as the Pirate Warrior or the aggressive version of the Quest Rogue. Plus, it gives a draw to help fiding Elementals or "combo" pieces. Put one instead of N'Zoth, the Corruptor or Shadow Strike.
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.