In the Series Edit
At the end of Death Masks, Nicodemus tossed the coin in front of toddler Harry Carpenter just as Harry Dresden was walking up to the baby. Harry picked it up instead. Harry burried it in his lab within his circle under concrete.
Afterward, in books Blood Rites, Dead Beat, Proven Guilty, and White Night, a copy of Lasciel took up residence in the unused portions of Harry's brain, with the intent of tempting him into taking up the coin permanently and becoming a member of the Denarians. Harry could converse with it.
In White Night, Harry talks about temptation and choice with Lash. He suggests that maybe being inside of him has had some affect on her, just as she has had some effect on him. When Lash is taken back into the whole, she will be gone: die. He then plants the seed that Lash has free will, too and that she can choose for herself, have a live outside of Lasciel.
In White Night, during Vittorio Malvora's crippling attack of despair, when Harry and his friends about to be overrun by ghouls, and a bomb is about to go off, Lasciel speeds up his mind to effectively slow down time. She makes a bid for Harry to call the coin to him—he refuses. Lash concedes she has a choice. Then, she tells Harry that Vittorio Malvora is possessed by an Outsider. Because of the circustances of Harry's birth, he has the potential to wield power over Outsiders. Harry can shake off the spell and she will give him the opportunity to do so. The she walks away toward Vittorio.
Description / CharacterEdit
Forthill describes her as "The Seducer", and she's also called the Webeweaver and the Temptress. It seems odd to Forthill that Nicodemus would want to free her (by giving her to Harry) since she does not follow Anduriel's lead.
Backstory / DetailsEdit
The sigil on her coin is a "vaguely suggestive of an hourglass."  One can assume from that, and the nickname "Webweaver", that she has a motif resembeling a black widow spider. Her coin was supposed to be in secure starage in a Chilean monastry. Yet, Nicodemus had it to toss at little Harry Carpenter.
Fan Speculation Edit
The name Lasciel most likely derives from the Latin, "lascivus", which can mean: wanton, playful, frisky, lustful, lewd, lascivious, etc. This is consistent with Lasciel's description as a temptress. It would then appear that "-el" was artfully affixed to the end, mimicking the Hebraic, theophoric naming convention of having a word for God in the name. For instance, one translation of "Uriel" is, "God is my light". Some credibility for this theory is that most angels in popular cultres have names that end in -el, and The Dresden Files convention of mixing and matching languages.
A much less likely origin of the name could be the Italian "lasciare", most often meaning "to leave (something behind)", which is used in one of Dante's most famous lines in his Inferno: "lasciate ogne speranza voi ch'intrate"/ "all hope abandon, ye who enter".
Lasciel's presence may be hinted at in the end of Ghost Story. According to Uriel, Harry was cheated out of free will by a Fallen who, in addition to having millenia of experience with human behaviour, knew Harry and his foibles well. However, this could be attributed to any number of the Order of the Blackened Denarius (e.g. Anduriel, who is depicted as a sentient shadow), or even other Fallen who have been inconspicuously watching Harry. This theory is given credability by the author's statement that Lasciel made an appearance in Ghost Story, but under a different name.