Why Attack Arctus Tor?

[Froklsnt]

The most likely identities for "that which stalks us all" are death, time, Nemesis, the persistent threat from the Outsiders, or a cycle of inescapable events (inevitability).

Considering that all of those could be combined into one, I'd say it's all of them.  Nemesis and the rest of the Outsiders present a persistent and inevitable threat to life itself.

In other words, The End.

Or put another way, Empty Night?

[raidem]

Agreed, but I don't see it as natural death or death separate from the cataclysmic threat that the Outsiders pose.

[Rasins]

I think Griffyn612 has the right idea. What I want to think about further is what Arjan brought up, what did Lea mean by "what stalks us all."

This reading makes sense to me, and strikes me as the most obvious interpretation. Lea tried to struggle against Nemesis on her own, but was unable to win the fight without help. But the struggle alone doesn't seem like the offense that Mab spoke of in Changes. So what was the action that Nemesis was attempting to cause Lea to perform? A simple, small action of verbal challenge, a simple questioning of Mab's orders would probably be enough to fit the bill, that's most likely.

But what if we think bigger? What if "I thought I could overcome what stalks us all" refers to death? That phrase is certainly a common motif of death. And the goal of vanquishing death is already held by Cowl (according to Kumori anyway), who provided the athame to Lea via Bianca. Lea would be in a position to potentially make some type of move against Mother Winter, who is the closest personification of "death" in the DV so far. And that would certainly qualify as a challenges to Mab's authority, an action against Mab's word and will. If Lea recovered herself and broke down in shame before she actually took an action, then everything fits.

In thinking about this, I can totally see either Nemesis OR Death being what Mab is referring to.

However there is one, slightly less obvious answer.  When you think about Jim and his upbringing, he is a child of the 80's and 90's.

Therefore it's entirely possible that he's referring to ....

(click to show/hide)

And their relentless pursuit of late fees.

[Froklsnt]

Therefore it's entirely possible that he's referring to ....
And their relentless pursuit of late fees.

No, anything but that!

More seriously though, I was just being cute before when I said Griffyn612's summary of "what stalks us all" reminded me of "Empty Night."  But as I think about it, I think that may be the honest correct answer, the summary of all the candidates proposed. "Empty night" is the favorite curse of the WCV, but it had one other very prominent appearance, on the lips of none other than Mother Summer:

"Ah, yes. I see," Mother Summer said. "So many new futures unwinding."
"Too many bright ones," Mother Winter said sullenly.
"Even you must think better that than empty night."
Mother Winter spit to one side.

This conversation was brought on by Harry learning of Nemesis. This change brought on the possibly many bright possible futures, and apparently was a blow against "empty night" coming to pass.  We also have this quote from Mab, in CH:

"I am Mab. The stars will rain from the sky before Mab fulfills not her word."

It isn't a big leap to say that Mab has given her word to protect our reality from Outsiders. The consequence of failing that, an Outsider invasion, would put out the stars in the sky, unmake our existence as we know it. That is the known goal of Nemesis, bringing on this "empty night." That phrase is shorthand for the horrible apocalypse that would befall us if Mab failed to keep her word and safeguard reality.

So, when Lea said  “I grew too arrogant with the power I held. I thought I could overcome what stalks us all. Foolish." she means she thought she could prevent empty night all on her own. I had been playing with the idea that perhaps Lea attempted something more overtly pro-outsider, but I think I've been talked out of that. Nemesis doesn't seem to work that way, anyway. It seems more to encourage a being to take their desires further than is rational, and to allow them to overcome foundational rules, such as a fae lying. Lea has shown several times that she has the impulse to try and overcome Mab, to "take the lead." Their relationship is one of slightly mismatched rivals. She even goes into detail on the topic in GS:

"In all of winter, I am second in power only to Mab. Which she has allowed because I have incurred with it proportionate obligation to her. She is my dearest enemy, but even I do not owe Mab so much..."
My godmother was what she was, a being of violence, deceit and a thirst for power... and believe me, if Lea had been the high priestess of murder, blood lust, scheming and manipulation, then Mab was the goddess my godmother worshiped. Come to think of it, that was probably an apt description of their relationship.

Lea's statement could be read that she tried to overcome Nemesis on her own. Or a simple overstepping of bounds, speaking out of turn or making a decision without consulting Mab first. It could also mean she interfered beyond her station in the war with the Outsiders. Maybe even an action that might have eventually led to Mab's ouster, similar to Maeve's goals.  But unlike Maeve, she became self-conscious of the change within herself, and recognized it as a problem, perhaps she even knew it was Nemesis. And so she went to Mab.