"Mandrake flesh is a narcotic. If he only had one bite, he'll probably be okay in a few hours--wait."

Twilight holds up a hoof in a 'wait a second' gesture, then casts a simple light spell to illuminate the hole--which, as it turns out, isn't merely a hole, but a tunnel running in a roughly east-west direction.

"This is the path to Tartarus. The mandrake was nesting right on top of it!"