An Edger was a predator that could only exist in two-dimensional environments and could not easily be destroyed. It broke through into other dimensions in order to feed but the energy needed to carry out the capture often left it vulnerable while it rested before feeding.
In 2012 an Edger managed to breakthrough into the real world via Christmas cards which it used to kidnap and abduct a family during their Christmas festivities. When the Doctor was abducted he was able to use his sonic screwdriver to create portals between the cards and free the family while the Edger rested before escaping back to the real world and destroying its route by burning the cards. (COMIC: On the Cards)