Somnambula is an antagonist from Generation 1 My Little Pony. Like a good number of antagonists in that particular canon of MLP, she was a wicked, cunning and treacherous individual with a surprisingly dark backstory - being a false immortal who drained the youth of others, so as to keep herself both young in appearance and powerful in her dark arts.
She was voiced by Jane Curtin.
Somnambula is an evil witch whose powers are stronger when she is younger. She has an canary named Kyrie whom she holds prisoner. She makes Kyrie sing to attract the ponies in a trance. As soon as Somnambula was younger she creates a magical circus and leads the ponies to it. She takes away the youth of the Earth and pegasus ponies to make her younger and the youth of the unicorn ponies to make her powers stronger and stores them in a crystal.
She also created illusions of the Big Brother Ponies' fantasies to keep them distracted. Kyrie aids the Baby Ponies and Spike in rescuing the other ponies. Seeing Buttons in agony, Slugger destroys the crystal causing Somnambula to rapidly age back to an old woman. The circus disappears and the ponies regain their youth.
Powers and Abilities
As a witch she has a variety of ill-defined magical abilities, most of which are designed to help her weaken and/or trick younger beings so she can steal their youth - which she requires in order to reverse her own ageing process, in effect becoming immortal (yet dependant on other being's youth to stay so).
She is also known to be stronger when younger, thus when she loses her youth her magic also suffers - like most other "false immortals" she will inevitably shrivel up and die without a means to steal life from others, which is her ultimate fate after her crystal was shattered.