Hcor'uk the Colossal's Portable Handheld Trebuchet of Walloping (Patreon)

Weapon (flail), very rare (requires attunement)

This flail has the two-handed, heavy, and reach properties. Attacks made with the flail deal an extra 1d8 bludgeoning damage, or 2d8 if the target is a structure. When you roll a 20 on an attack roll made with this magic weapon against a Huge or smaller creature, that creature must succeed on a DC 16 Strength saving throw or be knocked prone. 

The rope and boulder that make this weapon are magically attached and can’t be separated. The boulder weighs 200 pounds but is effortless for you to lift once you attune to the weapon.

A clan of ogres led by Hcor'uk, an ogre nearly twice the size of his kin, once took control of the walled city of Rissen. Hcor'uk slung boulders as if they were pebbles with the aid of a magical rope he'd stolen from a great wizard. One after another, he hurled the great rocks into the city's stone walls, tearing it down further and further with each passing moment. He used the fallen pieces of wall as more ammunition, adding insult to injury, and continued the assault to breach the city's limits. He was a siege crew all in one creature, and the city lay powerless before him....

Weeks passed, and the city fell to ruin.

Countless were slaughtered, and only a few lucky souls were able to escape the ogre's unending onslaught. It was left to a passing band of adventurers, ones not unlike yourselves, to save the city from Hcor'uk's grasp. Two days after entering the city, when almost all hope was lost, the band of adventurers climbed back over the fallen wall's remains, victorious. Behind them they carried the same rope once used to lay ruin to the city, but wrapped within it lay the severed head of Hcor'uk, the Colossal.