Sailor Moon's Adventures of the Black Cauldron is the season finale of Season 3 of the Sailor Moon's Adventures series.
Two-Face steals pure hearts from the people of Prydain. Chibi-Usa is abducted by the recently resurrected Negaduck, and her pure heart is stolen by Mistress 9, having taken over Hotaru's body, which allows her to awaken as the Messiah of Silence.
Mistress 9 and the Horned King use all remaining Daimon eggs to create a powerful energized shield of evil minions around Castle Annuvin. Sailor Moon is abducted, as she is the holder of the Holy Grail. After witnessing Sailor Pluto ultimately sacrificing herself to save them by using her forbidden ability to stop time, Sailor Uranus and Sailor Neptune destroy a hideously mutated Horned King with their talismans, the Deep Aqua Mirror and the Space Sword.
Sailor Moon is tricked into giving Mistress 9 the Holy Grail, which she uses alongside the Black Cauldron's evil magic to summon Unicron and his Cauldron-Born hordes to earth. Hotaru regains consciousness with the assistance of Sailor Moon, destroying Mistress 9 with the sword of King Gwydion and fully regaining Sailor Saturn's powers at last. Sailor Saturn herself appears before Sailor Moon with Gurgi to ultimately sacrifice herself and destroy Unicron once and for all with her destructive powers, but Sailor Moon gains the help of the other six Sailor Guardians and Taran's group to become Super Sailor Moon for the last time, and save Hotaru, who is reborn as a baby immediately afterward.