Derela attacked Saffron, and then impersonated her, announcing a talent competition. She then hypnotized everyone around (including Molly and Melvin) into taking part. She then absorbed their energy at the competition.
Seeing them perform in front of an empty audience made Serena and Luna suspicious, and Serena transformed into Sailor Moon to fight her. Derela revealed her true form and froze Sailor Moon, but Tuxedo Mask arrived and shattered the ice. Sailor Moon destroyed Derela with Moon Tiara Magic, breaking her spell.
Derela's name is likely a play on the name "Cinderella".
Instead of saying Moon Tiara Magic, Sailor Moon says "Moon Tiara Vaporize!" Followed by "I banish you to the Negaverse!"
Derela is one of two characters (technically three) whose death is referred to as banishment to the Negaverse". The other is Queen Beryl, who was combined with the Negaforce as Princess Serena destroys her with Cosmic Moon Power.