Ghost Guard is a side quest in World 1 - Kanterbury Forest.
Quest Gameplay
In 1-5 Ancient Ruin Entrance, there is a grave north of the locked box. Interacting with it will bring out a ghost woman who will tell the player that she wants the attention of another ghost and wants the player's help with it. This unlocks the sub-stage Forest's Treasure.
Upon entering Forest's Treasure, Ghost Guard Oralie informs the player that there are several ghost guards patrolling the area. The player has to avoid their sight range, indicated by a quarter circle in front of them, lest they be caught and automatically return to the nearest elevated platform.
Once the player goes through all the patrols, they will see the treasure - a sword in a stone. Pick it up and the ghost guard will panic. The female ghost will then come out and drag him away, completing the quest.