The Golem is located within the Lihzard Temple which is deep in the underground Jungle. You'll need a key from Plantera to enter. Within the temple you'll find alot of traps , statues , chests with power cells which you need to summon the actual boss at the end of the Temple to get amazing loot.
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Golem is one of the four bosses added in 1.2. It is summoned by inserting a Lihzahrd Power Cell...