The Adventurer's Sword is an ultra-rare pet accessory in Adopt Me! that is part of the Hero's Costume, which costs 250. However, the Hero's Costume is on rotation, so it is not always available in the Hat Shop, but this pet accessory can also be obtained through trading.
The Adventurer's Sword features a gray sword with a black hilt and dark red details on the blade and hilt. This pet accessory goes on a pet's back.
The Adventurer's Sword as seen on a Dog