As one of the ten Origin Devils, Shadow, upon being released, instantly drew the attention of everyone from both the Order Faction and the Dark Faction.
Shadow's true form is essentially non-existent.
The form it presented before John was actually a projection from the Dark Realm onto the Order Continent.
Shadow doesn't have a fixed form because it is an incarnation of the dark power from the Dark Realm.
This dark power can exist in any form.
The moment Shadow successfully broke free from the seal, it immediately focused its attention on John.
As one of the ten Origin Devils, it could naturally sense John's hostility towards it.
Moreover, in Shadow's view, no other being around John posed any threat to it except for John himself.