The boss of the Spirit's Grave is Pumpkin Head. This boss has two distinct phases. In the first phase, Pumpkin Head will float around the room, occasionally dropping smaller pumpkin enemies. Your goal is to attack its head with your Wooden Sword. After taking enough damage, its head will detach and fly around independently.
In the second phase, the body will remain stationary while the head flies erratically. You must now use your Power Bracelet to lift the body and throw it at the flying head. This will stun the head, allowing you to attack it with your sword. Repeat this process until Pumpkin Head is defeated. Be careful to avoid the flying head and the smaller pumpkins it drops. Once defeated, you'll receive a Heart Container and the first Essence of Time, the Eternal Spirit.