Telaendril joined the Dark Brotherhood when her father had hired the Brotherhood to murder her. Somehow, she managed to escape from her would-be killer Lucien Lachance, and murdered her father instead.
She was then offered a chance to join the Brotherhood, which she accepted.
Telaendril is one of the members of the Cheydinhal Sanctuary who must be killed during this quest.
Lucien: "Actually, Lucien did not try to recruit me. Not at first. He tried to kill me. My father wanted me dead, and he hired the Dark Brotherhood to do it. I escaped from Lucien, and paid my father back for his treachery. Lucien came to me again that night with an offer I just couldn't refuse."
Night Mother: "Our Lady is surely an ancient and powerful elf. Someday, when I am Listener of the Black Hand, I will look upon her beautiful and unholy form!"
Sanctuary: "In truth, I miss the towering elms of my native Valenwood. But I was banished from there, for deeds I don't care to discuss. This is my home now."
Sithis: "Long ago, in age now forgotten, Sithis came to the Night Mother and begat her five sons. So you see, his love gave birth to the Dark Brotherhood."
Rumors: "Oh, I shouldn't...well, all right. Gogron had a pet rabbit as a child! He petted the thing so hard he crushed its skull! Ha ha ha! What a brute!"