I think you get it on a certain quest. You're supposed to get it when you are level 28. Halston Balestorm usually gives the new card quests. Hope this helped!
I'm level 35 but I don't have the "tempest" spell. How do I get it??
Go to Halston Balstrom and make sure that you have the Storm Elemental spell, it is a requirement to proceed to the Tempest spell. You should have completed some of Marleybone as well to complete it. Good Luck!
Hope to see you all in the Spiral! Nathan GreenEyes 120 PvP Commander Main
Nathan GreenEyes 110 First Max
Ryan HexPyre 104 Always Dying
Nathan GreenEyes 75 Hot in more ways than one (lol)
In order to earn Tempest, you must first have gotten your Water Elemental minion at level 18. Did that? Good.
Next, you have to finish Krokotopia and get the spiral key to Marleybone before you will be able to finish the side quest for Tempest.
At level 28 Halstrom will summon you for a quest that starts off with you getting water from the fountain in Regent's Square, Marleybone; but if you haven't finished the main storyline in Krok, you won't be able to proceed. After you get the fountain water from Regent's Square then you have to defeat the Krokotopia boss, Krokenkamen. (Krokenkamen is the boss inside the Throne Room of Fire instance. Yes, you'll have to repeat that instance.)
So. Finish your main quest line and get yourself a spiral key to Marleybone, then you will be able to get your Tempest spell.
Here's a list of all the Storm school spells and how each is obtained:
PvP Commander 
Main
First Max
Always Dying
Hot in more ways than one (lol)
