Hades' Ember is a magical blue weapon owned and wielded by the god of the underworld Hades, and later given to Mal. It possesses great magical power that won't do everything that works for her, despite Mal being "half-Hades". When Hades was sent to the Isle of the Lost with the other villains, he managed to hang onto the ember. It is said to be more powerful than Maleficent's Scepter.
Role in the film
Hades' ember is first seen being hold onto by Hades. While attempting to escape the Isle of the lost, he blasts Jay, Ben and Carlos and he drains Mal's magic. After Audrey steals Maleficent's staff and the Evil Queen's crown from the Museum of Cultural History, Mal knew that she'll need the power of her estranged father's ember to overpower her mother's stolen staff. So she and Celia Facilier went to her Father's lair to get it. By the time they got there, Hades is fast asleep on the chair and they try to use the opportunity to take the ember without him noticing, but Hades woke up the moment Mal placed her hand on it. Knowing that they were caught in the act, Mal asks Hades to give her the ember to save Auradon from Audrey, all while having the talk that she and her father should have had a long time ago. Before Hades gives Mal the ember, he tells Mal not to get it wet, or it will lose its power, and despite being his daughter she'll won't be able to use the full power of his ember; but Mal is willing to take her chances for Auradon. Just as Mal and her friends were leaving the Isle, Harry Hook and Gil escape the island prison. This causes Mal to drop the ember, which gets caught by Uma before it hits the water. To get the ember back from her, Mal agrees to Uma's request of allowing the Isle's other kids into Auradon Prep.
Like Hades had told Mal she wasn't able to use the ember like he does, but joining its power with the magic within Uma's necklace does allow Mal to use the ember with Uma's help. When Mal kept being reminded of the hope that the other VKs have of being free from the isle and still being allowed to see their villain families, she knew that she'll have to come clean about her earlier decision to keep the barrier up for good, in which would prevent other villain kids from the Isle for good. To get back at Mal for lying and being the main reason why other VKs won't get the life they were promised, Celia takes the ember from her and drops it in water to make it useless. Despite the water rendering the ember powerless, Mal tries to use her dragon fire to dry it in order to save Celia from Audrey. Once Uma sees Mal struggling to dry the ember she uses her necklace to give the ember back its power.
After Mal uses the ember to defeat Audrey it places her in a death-like state, she remembers what her father had told her when he gave Mal his ember. Knowing that she alone can't use the ember to save Audrey, Mal convinces Ben and his parents to allow Hades into Auradon so he can use the ember's true potential to save Audrey; once she gives it back to him. Before Hades is brought back to the Isle, he secretly gives Mal back the ember.