Cross

The cross is an item in The Adventure of Link.

Location
The cross is found in the sixth palace, Three Eye Rock Palace; unlike the other items in the game, it is guarded by a mini-boss, Iron Knuckle on a jousting vehicle (a similar opponent appears first on Island Palace as a boss). After it's defeat, Link gains a key (although he doesn't need it anymore due to the Magical Key) and proceeds to take the item from a spot.

Uses
The cross, while useless in the dungeon where it's found, allows Link to see the dangerous blue-colored Moas that infest the graveyards in East Hyrule, the town of Old Kasuto, and the Valley of Death (which itself serves as the path to the Great Palace). It should be noted that, with a lot of skill, it's possible (albeit difficult) to survive those places without seeing the invisible enemies.