Depends on if you're a legend (member) or not. Irregardless, you have to /join xancave.
Complete all of warlic's quests. Afteryou face xan, then go back to the library. Warlic
will have a new conversation button, actually two. You want to click "become a
pyromancer". In the new conversation bubble, you want to select "blaze token quests".
Then, depending on if you're a legend, select either "shurpu blaze tokens", for 2 blaze
tokens per day, or "member: shurpu blaze tokens", for 3 blaze tokens per day. Next, you
want to go to the room before where you faced xan. Next, you want to attack the shurpu
ring guardian. Presuming you beat it, you'll recieve "guardian shale" temporary item.
Return to the library, turn the shale in, to warlic. At which point, you'll recieve either
2 or 3 blaze tokens. Keep doing this, once per day, until you have 86 blaze tokens. Then,
once you have 86, speak to warlic only this time, rather than selecting blaze tokens,
select pyromancer shop, choose the top one and press "buy". Congratulations, you now have
pyromancer class. You'll, of course, need to rank up in order to get all of the skills.