Big Spell Mage is a new archetype introduced in the Kobolds and Catacombs expansion. It is a more control based deck that uses higher mana spells which allows for Dragoncaller Alanna to produce a boards filled with 5/5 Dragons.
Kobolds brought along a few cards that can help support this archetype. Arcane Artificer is a solid defensive card that can help give you some armor when you are trying to stabilize in the late game. Raven Familiar brings back the concept of the Joust mechanic, and synergizes with your higher cost spells. Dragoncaller Alanna, as mentioned previously, is one of the reasons to play this style of deck. However, you will have to be weary of board clears and try to bait them out prior to playing her on the board.
Big Spell Mage got some more help in The Boomsday Project. Shooting Star is a strong potential board clear, Celestial Emissary boosts your spell damage, and Astromancer can give you a strong minion if you are carrying a large hand.
Check out an in-depth look at this deck type with mulligans and play strategy via our Big Spell Mage Deck Guide!