The Gargantuans

For millions of years in Isen's history, the planet was dominated by enormous monstrosities. 
Size and ferocity were the sole qualities that determined rule, and all smaller species, including humanoids, were given a choice to either cower in servitude or perish.
None were larger or more ferocious than the dragons.
In 11,190 BC, a pair of Surge wielders who were also powerful sorcerers waged battle against Kak'tinar, the self-proclaimed Dragon Emperor. The sorcerers were slain by his monstrous claws, but not before securing a few drops of his fiery blood, and using it to implement a powerful spell.
Over the days that followed, Kak'tinar died an agonising death, as he was torn apart from the inside by the sorcerers' spell. Once he had passed, a wave of magical energy burst forth from his carcass, quickly covering all of Isen, and ensnaring all monstrous creatures it touched. 
Then, every dragon and giant creature suddenly vanished from Isen.