Constructs, Part One.
Note that it is, in fact, immune to its own Stunning Burst. And that its mace does bludgeoning damage (not thunder).
“The Adamantine Golem is a treasured tool of war. It requires extreme expertise to create and maintain, and only the wealthiest of individuals can even hope to get the parts used to create one.
This golem is widely considered to be worth the effort to produce. It is a formidable enemy on the battlefield, so much so that it could lay siege to and level a town on its lonesome. Some say that an adamantine golem could easily do away with an adult dragon, and they might be correct.”