The undead? That they hid in dark caves guarding hidden Peruvian treasure!! Cris thought to himself. But since he was back at Hogwarts and Defense Against the Dark Arts class, he decided to remain on topic.

Raising his hand he gave his answer.

"Professor the undead are otherwise zombies or the Inferi. They are the dead who are brought back to some semblance of the living used mainly for the sole purpose to harm others. I guess they can be considered dark creatures, merely existing because a necromancer brought them back from the death ."