Earth Elemental Monolith - CR 28 (Gargantuan Elemental)

"In the closing years of the terrible wars between the Eschara and the Emendi, anything went, as each side began to realise that there would be no glorious victory or comfortable state afterwards. And so, in the days before an undocumented magical catastrophe forged what would later be called Zaepheri'Criz [Desert of Blades], their arch mages unleashed ancient beings of terrible power upon one another, hoping, despite the cost of calling them, that each might be the thing that finally brought them to some semblance of a victory..."

From "The Ancient Mage Wars" by an unknown Iradechian historian.

"Biggest bloody pile of rubble I ever had to put down. Would make a lovely feature in some bloody Aelwyn garden though. Probably."

Brundor Trullslayer, Vengeful Cleaving Axe of Thorduin.

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I'm pretty sure Elemental Monoliths first appeared in 2nd Edition AD&D's beautiful Al'Quadim setting, as a result of the Unleash Monolith spell (It exists in my 5e games, though as a ritual that requires the lending of  9th level spell slots, potent elemental magics and a bum load of residuum), and whilst bored, I decided to stat up the mostly commonly seen (in my experience) one of the four - the Earth Monolith. 

This thing is basically a living hillside, that wants to smash you and everything else to smithereens...and is entirely capable of doing so. If your GM uses one, you have probably really upset them...or made them very happy (if it's under your control). 

It was at this moment that Ser Belvegrath wished he'd taken up bloody carpentry like his father. Ah well *splat*