When Alex and Conner try to gather the magical items they need to make the Wishing Spell work, one "ingredient" is the dagger the Little Mermaid got from the Sea Witch to kill the man she loved.
"A saber from the deepest sea, meant for a groom's mortality."
It was already used by the twins' father and disposed of in the Thornbush Pit. The Seafoam Spirit tells them where to find it and asks them to destroy it after they use it for the spell.
It is retrieved by the twins with the help of shell necklaces to protect them from the Thornbush Pit's magic. They recognise it at once: "It had a curved handle that was made of coral and bits of shell, and a long blade made of bright sea glass."