Mandrakes are faeries, probably a rare type of sprite, that mimicks a simple plant, however, when pulled up they appear to be like a stray sod or toad with a plant sproting from them. They will wail (much like a banshee) and either cause one to go deaf when they are young, or die when the mandrake is full grown. Thus it is advisable to wear earmuffs when harvesting mandrakes. Their plant has purple flowers, and they are very deadly. Found in fields and woods, these creatures move from time to time and dig up a new home, and often (though disgusting) feed on worms and grubs.
Mandrakes are similar to the mandragora, a monstrous creature in folklore.
Mandrakes are in the Harry Potter book, The Chamber of Secrets. Like the mandrake from the Spiderwick Chronicles, they possess a horrible, bloodcurling scream. Banshees also exist in the Harry Potter series, and they scream also.