To obtain the Magma Vein, you have to ask around Tzhaar before a random NPC will reveal where to locate it. They will say the Magma Vein is found only in the deepest sanctum of Tzhaar. Find one of those Guard Tzhaars and Speak with them. Mention the Magma Vein and they will allow you in... under one condition.

They need a taste of Karamjan Rum. Head Beyond the volcano and go to the bartender in Karamja and buy one Karamjan Rum. Attempt to go indoors Tzhaar, but the rum will burst, doing a small amount of damage. Go back tot he bartender and ask him about the ingredients in the rum. He will mention that it will burst when exposed to intense heat. Ask him what would someone to perform should they"for a reason" desired to bring the rum into the volcano.

He will say that you must"ice" the rum down, so that it will not explode in warmth. Visit the icefiends on Ice Mountain, and kill you with the rum in your inventory. The rum ought to be chilled and prepared to bring in the volcano. Bring the rum to the guard and he'll let you into the inner sanctum of Tzhaar. You'll locate that a Magma Rock, which you ought to mine to get the vein.

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