If the only cards your opponent controls are Defense Position monsters, this card can attack directly.
Rulings
Individual Card FAQs
If there is a Spell or Trap Card on your opponent's side of the field, "Black Tyranno" cannot attack directly.
If there is an Attack Position monster on your opponent's side of the field, "Black Tyranno" cannot attack directly.
If your opponent has only Defense Position monsters on his side of the field, "Black Tyranno" can attack directly even if some are face-down.
If an effect resolves during the Battle Step of "Black Tyranno"'s direct attack that results in a Spell Card, Trap Card, or Attack Position monster being on your opponent's side of the field, a replay occurs.
If your opponent's only monster is in face-up Defense Position, but it is equipped with your "Ring of Magnetism", "Black Tyranno" must attack that monster if "Black Tyranno" attacks.
EARTH|
Level 7|[ Dinosaur / Effect ]
ATK 2600|
DEF 1800