The key seems to be the location of the headaches. He's got two places he normally has headaches.
1) Between his eyes/bridge of nose
2) His temples.
I think the key headaches are the
temple headaches. I think the
temple itself has been emphasized several times.
I'm currently thinking that Mab hasn't altered Harry's memories, like we thought. I think it was the Parasite, who I think is Lash. I think Mab and Lash have been working together longer than we thought.
The link to the parasite is through the "
ice pick" descriptions. Those link to the
temples, with link to the
headaches since Lash disappeared.
There may even be a link to the damage Harry suffered at Lash's 'demise', and other mental manipulations we've seen.
Mental/Memory ManipulationPG - pg. 112 (Molly manipulated Rose's head; but what about Harry?)
Worse, there were what looked almost like burn marks on her temples—small, black, neat
holes, as if someone had bored a hot needle through the skin and skull beneath.
...
My head started pounding and I pushed the Sight away. I leaned my hip against the wall
for a second and rubbed at my temples until the throbbing subsided and I was sure that my
normal vision had returned.
PG - pg. 134 (Titania manipulated Lily)
“I know,” Lily said. She lifted her hand to a temple and closed her eyes in a faint
frown of pain. “But…’’ She shook her head and fell silent again as Titania’s binding sealed
her tongue.
PG - pg. 250 (Molly manipulated Nelson's head)
And each of his temples bore tiny, neat holes, sharp and cauterized, as if by a laser
scalpel.
Birth of LashWN - pg. 309
“For an inhumanly brilliant spiritual entity, you can really miss the freaking point.” I
poked a finger at my own temple. “Think. Maybe you don’t have to be Lasciel.”
WN - pg. 442 (Bob examining Harry after Lash's sacrifice)
“I think that’s where the damage is. I mean, I’m looking at you right now, and your head
has been riddled with tiny holes, boss.”
WN - pg. 444 (Bob talking about Lash existing off of Harry)
“So it was running off my soul? Like I’m some kind of battery?”
SmF Headaches, First TarpSmF - pg. 46 (When Mab took the rod; what else happened?)
Mab’s frozen-berry lips lifted in a silent snarl, and the world turned into a curtain of
white agony that centered on my eyes. Nothing had ever hurt so much. I fell down, but I wasn’t
lucky enough to hit my head and knock myself unconscious. I couldn’t move. I couldn’t breathe.
I couldn’t scream.
Then there was something cold beside me. And something very soft and very cold touched my
ear. I recognized the sensation, from the far side of the pain. Lips. Mab’s lips. The Queen of
Air and Darkness placed a gentle row of kisses down the outside ridge of my ear, then sucked the
lobe into her mouth and bit down quite gently.
SmF - pg.273-274
On the opposite side of Ivy, Rosanna launched more traditional lances of flame from her
open palms, much like the ones I—
—a savage pain went through my skull for a second—son of a bitch—
—but Ivy dispersed them with delicately applied wedges of air, intercepting each burst of
fire far enough short of her body to prevent the bloom of heat as they died from scorching her
—though the two more physical Denarians who strained to force their way past the barrier of
snapping sparks that formed whenever they tried to get close had far less luck.
SmF - pg. 312
Pain stabbed me in the head, ice picks plunging into both temples. I flinched and
doubled over. Blasting rod. Familiar words. I fought to summon an image of what went
with the words, but I couldn’t find anything. I knew I had a memory associated with those
words, but try as I might, I couldn’t drag it out. It was like a shape covered by some heavy
tarp. I knew an object was beneath, but I couldn’t get to it.
Turn Coat Headache, Another TarpTC - pg. 1/9
The summer sun was busy broiling the asphalt from Chicago’s streets, the agony in my head
had kept me horizontal for half a day, and some idiot was pounding on my apartment door.
...
I pulled the door most of the way shut and sank down into the easy chair by the fireplace,
where I could rub my temples and try to think.
TC - pg. 267
A long table in the middle of the room was currently covered by a canvas tarp, and the
floor at the far end of the lab had a perfect circle of pure copper embedded in it. The
remains of several differently structured tracking attempts were scattered on the floor
around the circle, while the props and foci from the most recent failure were still inside it.
TC - pg. 272
She chewed on her lip. “What are you going to do?”
“I . . .” My voice trailed off. I sighed. “I have no idea.”
Mouse pawed at my leg and looked up at me. I bent over to scratch his ears, and
instantly regretted it as someone tightened a vise on my temples. I straightened up again in
a hurry, wincing, and entertained wild fantasies about lying down on the floor and sleeping
for a week.
The Parasite PainCD - pg. 164
One second, my scalp was tightening up as two separate ice picks dug into my skull in the same places
they always did, and the next the pain was utterly gone.