So. Your name is Lasciel. You’re a fallen angel imprisoned in a blackened denarius. You’ve just learned that the Harry Dresden has outsmarted your mental construct. You’ve lost face with your fellow evil cohorts, and Hell hath no fury like a Fallen scorned. What do you do next? Why, you make the wizard as miserable as possible, and then you kill him. How does one accomplish that? Here’s my theory.

I’d like to make the case that Lasciel has been whispering thoughts into Harry’s head since Turn Coat. Harry’s reliability as a narrator sometimes comes into question, but his narration style doesn’t change. Up until Turn Coat, I can find no instances where his inner monologue includes an unnamed voice whispering in his head. Yes, Lasciel’s coin whispered to Harry at the end of Death Masks, Lash had mental conversations with Harry, and the Merlin gave psychic instructions to everyone at the end of TC, but in each of those instances, Harry is consciously aware that his thoughts aren’t his own. 

I believe it’s possible for something to be inside Harry’s head undetected. Molly got a good look around in there in SmF, but since then, her experience has been limited to penetrating Harry’s defenses and doing a Darth Vader impression. And at the end of TC, Harry mentions all of the younger wardens getting brain checks, but never mentions that he himself is examined.

I don’t know how Lasciel got into Harry’s head, but I’m guessing that the process was painful for Harry and gave him migraines. The migraines have disappeared by the end of TC, which is after Harry has heard the voice for the first time. Harry and Bob are discussing what shagnasty did with Thomas during TC.

“He’s alive,” I said quietly. “Or at least I’m going to proceed on that assumption.
Bob somehow managed to look baffled. “Why?”
Because you need your brother to be all right, whispered a quiet voice in my head. “Because anything else isn’t particularly useful in resolving this situation,” I said aloud.

And the voice causes Harry momentary despair.

Then there are four separate instances of a voice whispering in Harry’s head during Changes, not including the one that Molly erased. The first time happens when Harry is going after the RC vamps in his office, right before he decided to forgo steath mode and attack the vamps.

And why, exactly, a voice somewhere within the storm in my chest whispered, should I be hiding from these murdering scum in the first place?

This could be amusing for Lasciel. Will this idiot act on what I say? Why yes, he will. Muahahahahaha.

In the second instance, Harry is running out of options and contemplating possibilities. Option one, get friends killed trying to save Maggie. Option two, do a smash and grab against the whole RC. Option three, become a tardbeast.

But Maggie might survive it. If I did it right—only to be orphaned again, in one way or another.
I felt so tired.
Maybe there isn’t a way, whispered a voice in the back of my head.
I snapped the water off and reached for a towel. “Screw that kind of thinking, Dresden,” I ordered myself. “There’s a way. You’ve just got to find it.”

And the voice causes Harry momentary despair.

The next instance isn’t mentioned in Changes, but results in Harry putting a hit on himself. Solid proof that one of the fallen is whispering in Harry’s thoughts. But even if he is going to die, there’s still more misery to be had. When Harry is contemplating becoming the Winter Knight, this happens.

It was the least evil path, I told myself. Whatever else I might have done would have turned me into a monster in truth. Because of Lloyd Slate, I knew that whatever Mab might say, she did not control her knight completely. Slate had defied her power and influence.
 And look where it got him, a little voice whispered in my head.

And Harry feels despair.

The last instance of voices in his head mentioned in Changes is when Harry is in the Erlking’s domain fighting the Ick and it doesn’t look like the Ick can be defeated.

It howled and lifted both arms above its head, ready to smash them down on my skull. I didn’t have much time to use my death curse, said the little sane voice in my head. 
And then another voice in my head, one far louder and more furious, screamed denial.  My few glimpses of Maggie whirled through my mind, along with images of her death—or worse—at Arianna’s hands. If I died here, there would be no one to take her out of darkness.
I had to try.

My interpretation of this is that Lasciel sees another opportunity to kill Dresden and takes it. And then Uriel intervenes because Mab wouldn’t be able to get to him if he died in the Erlking’s domain.

So there you have it. Getting into Harry’s head caused the headaches in TC, and Lasciel’s been getting her revenge by steering him towards suicide. 

Significant Replies
Vairelome's support summary  (Reply #63)
Vairelelome's analysis of Lasciel's (possible) "tags"  (Reply #80)  Jim's explenation of "tags"
Tie-in Thread "The Voices in my head" (Reference of all the times a disembodied voice spoke to Harry)
Here is another thread with many parallels

Back to the Molly as Titania suggestion. It makes as much sense as Molly being Mab.  If Molly was/is Titania, it explains why Harry has never seen her, because Harry recognizing Molly as Titania could possibly cause a time paradox. It's why Titania didn't kill Harry while he was defenseless at the end of SK after he had killed Auora.  Also, Mab knows/knew that Molly had a thing for Harry, so she wanted Harry as Winter Knight out of spite and

I'm also super glad that the scenario in the second olgaf link didn't happen to Harry, although I'm sure that Toot would have found it interesting.