Before the Groundshaker came to be, it was originally Scar's Ghost. It attempted to terrify Simba to the point where he doubted that he was worthy enough to rule the Pride Lands.
After Simba finally confronted his fears, doubts and weaknesses about being the ruler of the Pride Lands, the many Ghosts of Scar flew to the Savannah and coalesced into a giant Emblem Heartless known as the Groundshaker. It greatly resembled the Living Bone-Shaman symbiote, while it's riding on a quadruped Heartless. After revealing it's true form, Sora, his friends, and Simba confronted the giant Heartless. After a long battle, which raged all across the Savannah, Sora, his friends and Simba managed to defeat the Heartless and finally bring peace back to the Pride Lands.
The Groundshaker is a powerful Heartless. When it was Scar's Ghost, it could induce fear towards Simba and turn invisible. When it takes it's actual form, the Groundshaker gains a few new abilities such as creating shockwaves when it performs a stomp, can generate fire, and shoot lightning from it's eyes and various other areas.
If Sora manages to get on top of the Groundshaker, It's revealed that there's a shaman-like Heartless controlling the Groundshaker, on the creature's back. The Shaman can perform a punch combo so it can attempt to get Sora off.