The Empress was a vampire and a member of the Vampire King's court.
As a member of the Vampire King's court, The Empress enjoyed all the luxuries the vampire hive had to offer such as devoted servants and premium blood decorated with gold leaf. Marceline fought The Empress centuries ago when she was on a mission to protect the remaining humans from vampires. The Empress' dialogue in The Empress Eyes suggests that she met and hypnotized Simon Petrikov before he fully transitioned into the Ice King. She made him perform a series of degrading acts to toy with him. Marceline managed to defeat her and gain her powers of hypnosis and invisibility.
The vampires are resurrected after Princess Bubblegum removes Marceline's vampire essence from her. After a short period, the Vampire King and his court manifest. The Empress makes a case for resurrecting the vampire hive, suggesting that they conquer and enslave the local populace. When it appears that disagreements are breaking up the vampire court, The Empress parts ways with the group to go rebuild her army. She heads towards the Ice Kingdom.
The Empress wears a long black dress, with a golden snake that is wrapped around her waist and neck. She also wears a dark purple wrap over her eyes, with a pink gem in the middle. Underneath, she has large pink eyes, which see uses to hypnotize weak minded individuals and put them under her spell.
As a vampire with magical powers, the Empress can hypnotize people in order to control their every move. The golden snake wrapped around her waist and neck slithers up to the wrap around her eyes and pulls it up when the Empress wants to hypnotize people. This is shown in The Empress Eyes, when she tries to enthrall Ice King (though he is able to resist her powers). In addition, she is able to turn invisible at will, a power that Marceline inherits when she stakes her and claims her soul.
Against Sora's Team Edit
Once Master Xehanort resurrected the vampires by using a device to scan Marceline and revive the Vampires; they became twice as stronger, and became major threats than before.