Cage of Nightmares (D-BT04/0038EN) (D-BT04)

[Set] (This card remains on the field.)
This card's ability on the field cannot be nullified.
When a "Black Dragon" enters your field, put the top card of your opponent's deck into the drop zone.
When this card is put into the drop zone from the field, put up to one "Black Dragon" monster from your drop zone into your hand.