Gorm was the leader of a large tribe of ogres, the Thunderguts. They lived in the usual ogre way: peacefully raiding their neighbors and making off with their livestock and goods. Then the next season the neighbors would raid the Thunderguts and try to take their stuff back. It was just the circle of life.
Then one season the circle was broken when a large army of dwarves invaded the hills that the ogre tribes lived in. Gorm rallied the various tribes and fought off the dwarves. The ogre tribes had a huge celebration and partied for days. Unbeknownst to them, they had only fought off part of the dwarven army. The dwarf survivors rallied and attacked the drunken ogres who fought valiantly, but ultimately were all slaughtered except for Gorm. The large ogre repelled wave after wave of dwarven invaders, killing many while seemingly untouchable by the stubby weapons of the stubby creatures.
Eventually the dwarves decided to cut their losses and fled the scene of the battle. Gorm chased them a short distance but was too exhausted to follow for long. He staggered back to his village to try to save any of his people, but it was too late. There was no one left alive.
Ever since that day Gorm has been looking for the leader of those dwarves. All he knows is that the dwarves flew a banner of a stone fist and the leader wore a helmet with a broken horn.
He has heard of growing uneasiness between the elves and dwarves and has decided that his best chance at getting his revenge would be to ally with the elves. His goal is to build an ogre army to fight alongside the elves and kill every dwarf that stands in his way.