The Man-in-Stone is one of the NPCs you'll encounter while exploring the many realms of Black Myth: Wukong and this character has a unique problem; he's encased in rock. If you've arrived at the not-at-all unsettlingly named Fright Cliff in chapter two's Yellow Wind Ridge, you may have already encountered some rock guai, that is to say, enemies who are effectively a pair of legs holding up a massive rock.
It's all the fault of a nearby rock guai dwelling in a cave according to the Man-in-Stone, so you'll have to defeat it in order to help him. Completing this NPC's quest will get you one of the harder-to-come-by Transformation spells, plus open up a new vendor, so it's well worth your time. Here's how to help the Man-in-Stone in Black Myth: Wukong.
Once you arrive in the Fright Cliffs in the Yellow Wind Ridge in chapter two, it's relatively easy to track down the Man-in-Stone. From the Squall Hideout Keeper's Shrine:
Talk to him and he'll explain that he needs some help, namely, that he wants you to defeat a rock guai in a nearby cave that's creating all the stone enemies wandering around the area.
The rock guai in question is the Mother of Stones boss, but she's tricky to find in the maze-like passages of the Fright Cliff. From that same Squall Hideout Keeper's Shrine:
The Mother of Stones is pretty easy to recognise since she's a stationary golem with glowing crystals set into her. The boss won't move as you damage her, but she'll summon waves of rock guai that you'll have to defeat if you want to damage her undisturbed. There's no rush, though, so just take your time and defeat the enemies. Once beaten, she drops the Stone Essence key item.
Now you've got the Stone Essence you can return it to the Man-in-Stone, but it turns out he was tricking you. He pops out of the ground and calls you gullible for helping him claim such a treasure. You might notice, however, that now you can target him. Attack the Man-in-Stone and he'll become a boss. He isn't that hard to beat—he's just a slightly bigger version of the regular rock guai with a different moveset.
Once defeated, he'll hop back into the ground and you'll be able to absorb the Azure Dust Transformation spell that lets you turn into a Man-in-Stone and headbutt enemies. If you speak to the Man-in-Stone again, he'll explain that he can sell you some fine treasures if you return.
Rest once and return to him to browse his wares. The two most important items he sells are the Sobering Stone—which you can use to sober up the drunken pig NPC in the Fright Cliffs if you don't already have one—and the Gold Ridge Beast treasure.