'The vilest deed of Melkor'

"All those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves(...) This it may be was the vilest deed of Melkor, and the most hateful to Ilúvatar"
Tolkien, "The Silmarillion"