Elves who fail to control their addiction and succumb to it-- they change.

[his ears twitch a little uncomfortably, at the thought.]

Either they indulged too much, or they starved themselves until they couldn't help but give in and glut themselves on magic. It doesn't matter either way, they still... they lose control. They're slaves to their hunger, at that point, they consume so much of it that it literally grows from their bodies.