All forging is done at the Sor Forge. This can be found to the right of the spawn in the Nexus.

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To forge a legendary put a Legendary Sor Crystal at the top and the Shard at the bottom and click Forge.
Depending on the Shard you use it will give you a legendary from the table of that Shard. Certain items can also be upgraded using a Legendary Sor Crystal such as the Warped Worlds Staff. Put the item you are upgrading at the bottom and the crystal at the top. Alternatively, at a fee of 75,000 gold, you can put a shine and a legendary item craftable from a shard in order to re-roll it into any other craftable legendary item from a shard.

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