So, we like our heroes to be someone we can commiserate with. Someone we can understand and follow. I, personally, can’t imagine revenge because, well, I’m just not that type of person. But, I guess I can pretend to be someone who would seek revenge.
Which leads me to some sort of possession.
So – say the item (golden egg) belonged to a dragon. The dragon is often thought of as a magical being who can change shape from its scaly winged form to a human form. In this world, the dragons are second-class citizens. Used for their magical abilities, they are enslaved through capture of their eggs. The parent dragon is negotiated with and gets to see its young but, for some human users, not have it. The young dragon grows up in slavery, producing magical items or providing magical transport in a world that would otherwise be restricted to travel by cart or in some foreign/advanced culture, steam power. Sometimes the dragons provide the steam. The dragon is no longer a symbol of a powerful creature but a symbol of the controlling human’s power. Boy are they wrong.
They pissed off the wrong dragon daddy – stole his golden egg – and now he’s got control of the vengeful-minded under-mage (heroine) and he’s going to wreck havoc on those who stole his egg and the rest of the human population.

And now I can see writing that character. The vengeful dragon-mage combo who right a wrong and change the world.