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.
Background
The comb was originally 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 turns out so does Cedric. He came along with the Royal family aboard the Floating Palace in search of it and then use it to take over the kingdom of Enchancia. Cedric learned about it from a book of mermaids that Sofia also happens to have a copy of.
Trivia
Like the Amulet of Avalor and the Emerald Key, the comb is a powerful enchanted object wielded by a princess.
All three objects are also sought by villains seeking to use their power to take over their home kingdoms.
The comb and the Emerald Key have both been revealed as weapons in experienced hands, though the princesses bearing them do not use them as such.