The Caverns Below

The Caverns Below is a legendary rogue spell card, from the Journey to Un'Goro set.

Strategy
An effective way to complete The Caverns Below is to use return to hand effects such as Youthful Brewmaster, Shadowstep, Cheat Death, Vanish as well as cards that create copies of other cards, such as Mimic Pod, Sonya Shadowdancer and Zola the Gorgon. For a list of all cards with return and copy effects, see return to hand and copy.

You can also complete this quest using the Flame Elemental tokens generated by Igneous Elemental and Fire Fly. Not only do these quickly place minions with the same name into your hand, they generate 1 mana 5/5s once the quest is complete, and have Elemental synergy.

Crystal Core is one of the trickier Quest rewards to make use of. It has obvious synergy with cheap, low-statted minions like Bloodmage Thalnos or low-statted minions with strong effects or keywords like Elven Minstrel, Wax Elemental, and Vicious Scalehide. On the flip-side, it has severe anti-synergy with many high-cost minions, as most will be reduced in stats or gain little to no benefit. Low-cost charge minions like Stonetusk Boar and Southsea Deckhand are extremely effective with Crystal Core, which make up the bulk of the damage dealing and allows return-to-hand effects to remain highly effective. Other cheap token-generating minions such as Violet Teacher and Moroes can also work well with it.

Because the quest reward is a spell, Preparation is very useful for playing the reward earlier.

Lore
The crystals of are ancient  energy sources. The Crystal Core is a particularly powerful one, and represents infusing the affected minions with titan energy, comparable to the opposite of Old God corruption, "except that you look like a at the end instead of a tentacle monster.  A 5/5 Norse god."

Trivia

 * The enchantment aura provided by Crystal Core is similar to the enchantment aura Miniature, encountered in the Tavern Brawl Miniature Warfare.
 * The spell's visual effect consists of two card's effects reused: the first stage is Devolve's mist cloud effect, extended by Renounce Darkness' shimmering effect and some small sparks.

Patch changes

 * Condition changed from "Play five minions with the same name." to "Play four minions with the same name."
 * Crystal Core now reads "For the rest of the game, your minions are 5/5." (Up from 4/4).
 * Crystal Core now reads "For the rest of the game, your minions are 4/4." (Down from 5/5).
 * Condition changed from "Play four minions with the same name." to "Play five minions with the same name."
 * Added.
 * Added.