The Mermaid Comb of Merroway Cove is an enchanted hair comb with the immense hydrokinetic ability to control all of the seventeen seas and oceans.
The comb was given to Oona by her mother, much to Cora's disappointment. She was told it possessed great magical abilities but would not tell her until she was old enough.
It was said that Queen Emmaline and Cora only knew about the comb's magic but it turned out so did Cedric. He came along with the royal family aboard the Floating Palace to get his hands on it and use it to take over the Kingdom of Enchancia. Cedric and Wormwood, as sea creatures, tried to take the comb by force but failed because it had a protection charm placed on it. After capturing Oona, Cedric used his magic to try and break the charm and remove the comb from her person. He successfully broke the charm and removed the comb from Oona's person but Sofia intercept it and tossed it to Cora, who used its hydrokinetic powers to defeat Cedric.
In "Cool Hand Fluke", it is revealed that Cora has shown Oona how to use the powers of the comb and has led to Oona using it herself when she made water spout around the Mer-Tones while they played music for Sofia's family.