Undead is a generic term used in the Sailor Moon musicals to refer to the various mythological beings that appeared within the Dracul arc. Although the traditional meaning of "undead" implies creatures that are dead and yet acting as though they still live, in the musicals this group included creatures such as vampires, werewolves, demons, Muma, Youma, Yokai, etc., which were all said to belong to the underworld.
In the Dracul arc, one of the main conflicts was the one between the Undead and humans; the former accused the latter of overrunning and polluting the planet, leaving most of them trapped in the underworld. Many of them were also said to be jealous of human bodies, as they could not have physical bodies of their own. This brought the Undead to fight against the Sailor Senshi, as they were the protectors of mankind. According to the musical continuity, Sailor Moon's Silver Crystal was what maintained the seal of the underworld, thus keeping the bridge between it and the human world closed.
In Last Dracul Jokyoku, Undead Berserk and his Undead associates attempted to create perfect Homunculi to replace the human race. To this end, they needed a Pure Heart to give the Homunculi flawless souls. Berserk's first experiment created the Death Mannetjes, whom he viewed as failures because of their cheap souls. In the musicals, Berserk recruited the help of Count Dracul, who joined him with the intention to use the Homunculi's blood to feed his daughter as she refused to feed off of humans. Eventually, Berserk captured Chibiusa's Star Seed, then fused it with his growing Homunculi. This gave birth to the Samael Quartet, who were later used by Lilith of Darkness to fight against the Sailor Senshi in Transylvania no Mori and its revision.
It was revealed later in that musical that Dark Cain was the one who originally created the Undead; his master plan was to drive them into a war with humanity in the hope that it would bring about the end of the world, since the destruction of the Earth was the only way for him to finally be able to die. He considered the Undead to be his very own Homunculi and regarded them with contempt. In order to bring his plan to fruition, Dark Cain associated himself with Death Vulcan who in turn recruited Undead followers to fight against the Sailor Team.