Greetings!
Hmmm...what would one think should a maxed-out Great Wyrm decided to crush a huge city. The Dragon defeats the city's army, and slaughters the majority of the population. In addition, the Dragon enslaves a few hundred-thousand people as slaves. The Dragon has them toil and work to improve his mountain lair--for years. Once completed, he eats most of them. He keeps the rest on staff, permanently charmed/dominated as guards. The Dragon breeds the humans into half-dragons that are raised from birth to worship and serve the Dragon as their god and father.
The Dragon also enslaves the city's academy of wizards. The wizards, under their domination, are naturally willing to train the Great Wyrm in the art of Wizardry. Thusly, the Great Wyrm trains for years, and then advances on his own. Over time, he will not only ba a maxed out Great Wyrm, but also a 20th level Wizard. Or perhaps at 10th level, he decided to train for the next 10 levels as a Loremaster.
By this time, the Great Wyrm is awesomely powerful, and has developed his own nation of half-dragon children that love him fanatically, as he loves and cherishes them, and equips them well, and teaches them many great powers and abilities. The nation of half-dragons develop hundreds of wizards, all trained by their ancient father, the Great Wyrm. The powerful nation then marches to war, and conquers an empire. Gradually, the half-dragons reproduce, so you have a growing nation, with say the Great Wyrm also attracts his own mate, to start having clutches of baby dragons. Meanwhile, the half-dragon nation continues to grow in numbers, and power. The Dragon could shape-change into human form, and use his immense powers, and immense wealth, to further equip his armies with powerful magic, even while they spread out and conquer, and continue to bring the plunder of nations, and perhaps other dragon-hordes to his vast treasury, which in turn further enriches and empowers the Great Wyrm, who in turn continues to teach his children, and to train them, and equip them for conquest. Generation after generation this process continues!
Semper Fidelis,
SHARK