The Gift is a legendary mystical amulet that was the ancestral heirloom of the Cromwell family and it has the power to control anyone at will.
The Gift was created by 3 Cromwells and in turn can only be destroyed by three Cromwells. It is unknown which Cromwells crafted the Gift.
It was revealed when Marnie went back in time, she discovers that the Gift was inherited to Queen Splendora Agatha Cromwell, or Aggie Cromwell who happens to be Marnie's beloved grandmother. And also her father Marvin, cousin of the legendary wizard Merlin, was part of that same family possessing the power to control the magic of the Gift itself. She despised the Gift's power, and she locked it away for 1000 years.
The Gift was wanted by Silas Sinister and his group of powerful evil companions known as The Dominion. They wanted to use the Gift to have complete control over Halloweentown and use the creatures as slaves.
Marnie commands her genie friend Aneesa, her mother Gwen, and her brother Dylan to trap the Gift in the lamp and Marnie, Gwen, and Dylan combined their magical powers and cast a spell to destroy it.
At the end of the movie, It was revealed that Marnie did not destroy the Gift, but somehow transported it into the middle of a book that Dylan had on his shelf. Dylan is studying in his dorm and opens a book to discover the Gift hidden in its hallow shell. The book has a giant S letter on the side, revealing it to the symbolic seal of the Cromwell family and representation of the name of Queen Splendora.
It is uncertain what happened to the Gift afterwards.