Image imported from derpibooru 
Original Description:
 
Gusty facing down Grogar atop the mountain, to steal his power away and free Cloudsdale and the pegasi inside from his grasp. She’s meant to have an army of unicorns casting golden chains at the clouds but I didn’t want to do all that mess.
 

 
Taken from Hero.Fandom the legend of Gusty the Great goes as so;
 
“Thousands of years ago, long before the pony tribes united as one, the country that would be known as Equestria was a dark and forboding land where horrible monsters created by the evil emperor Grogar terrorized the populace so that they could fuel their creator’s power with the denizens’ fear. But then a brave unicorn named Gusty the Great rose up alongside her Unicorn Warriors to challenge his rule.
 
They encountered the evil ram atop the hill that would lead to Cloudsdale; The Unicorn Warriors shot magical beams into the clouds and wrapped around Grogar and pulled him to the ground, but the evil goat manages to break through the bonds as fear gave him power. The rest of the battle was unknown, but it was confirmed that she managed to steal away the Bewitching Bell, which was the source of Grogar’s power, and banished him from the land.
 
Afterwards, Gusty hid the bell within an enchanted cave atop Mount Everhoof, which was protected by a powerful forcefield and magical winds to prevent anycreature from getting to it.”
 

 
Grogar gains power through fear, making him one of the strongest and hardest to beat enemies in pony history, he is also the ‘Father of Monsters’, at least according to Discord but seeing as Tirek didn’t correct him we can assume some truth in the title.
 
There are significant differences between G1 Grogar and the History we are told in the G4 show, for example G1 Grogar rules over a land called Tambelon and captured unicorn ponies (and their friends), while the G4 Grogar is said to have resided in the clouds, in what either was or would become Cloudsdale, the pegasi capital. So I swapped the unicorn prisoners for pegasi.
 
Gusty and her warriors do succeed in beating Grogar, taking his bell and freeing the pegasi he trapped, but Grogars effects on the world linger long long after him, such as in the monsters he creates, and the damage he did personally during his reign. The largest effects being: the Wind-evils that fuel the fued between the pony tribes, and the destruction of Equestrias Sun and Moon.
 
Yep, I’m blaming Grogar for the reason pony power is required to move the sun and moon within Equestria. After all, what happened before ponies? Grogar was immensely strong, having fueled himself with the fear of the ponies he tormented, and he demonstrated this by shattering the heavens, destroying the natural order of the sun and moon, cloaking the sky in dark clouds, and shrouding Equestria in shadows.
 
And so, while Gusty rids the world of Grogars terror, the ponies still had to rebuild and make do in a world that was vastly different than the one before, monsters were creeping up on them in every corner and the unicorns scrambled to right the heavens. But Gusty’s part in the major story is over now.