A Magma Crystal is an object that can summon Magma Cubes and Blazes to do your bidding and generate a Lava source block, basically dumping lava. There are only a few dozen in existence. They can't be crafted, and they can only be found in the unexplored, sprawling dungeons underneath a Nether Fortress, where they are coveted and guarded by Blazes, Nethermen and Zombie Pigmen.
A Guy on the Internet won a free Magma Crystal for his vote for Chapter 1 of THaB - Death by Giant Accordion. The next vote was for Chapter 2, and the winner, A Guy on the Internet, won an Yggdrassil Sapling for his decisive vote of "How Not to Ride a Ghast."