I'm not sure it has ever been explicitly stated who assaulted Arctis Tor before Harry showed up. However, given what we later learn about Lea, there is one obvious candidate:
The Black Council
Specifically, one or more Denarians, on the orders of Nemesis.
At the time, all we knew what that Mab had Lea prison, in ice, and something was clearly wrong with her. She even told Harry not to free her. We later learn that she had been infected by Nemesis. She was infected way back in Grave Peril, and sometime after that, Mab figured it out and set out to cleanse her.
Given how powerful Lea is, she would clearly rank as a high-value target for Nemesis, so having Mab free her from Nemesis and spill her guts about what she knew is really bad news for the bad guy. They would definitely have wanted to free her, or else kill it, whichever they could. And since she was being held in the heart of Winter, the obvious weapon to use: Hellfire. Most likely, Thorned Namshiel (probably with help) tried to assault the fortress to get to Lea, but Mab showed up and fought him off. Even by himself, he could have done serious damage, especially since most of Mab's army was off on the border with Summer at the time.
When Harry has his heart-to-heart with Nicodemus later (Small Favor, maybe?) some of this is sort-of implied, but I don't think he comes right out and admits it. But, given the very small list of candidates that had both means and motive, I think we are supposed to have pieced together that Nemesis-backed Denarians are the only realistic candidate.