Not sure what it is, but Lash was already a passenger in Harry's mind when he was hit by the Elements of Harmony. This could be psychicscubadiver's explanations of why she didn't seem quite so Evil as she was described to be, or it could completely invalidate parts of books 7 & 8 in Dresden Fillies continuity.
Was she? False Masks takes place after Blood Rites, but Strange Friends is a bit more iffy. He's traveling though the Nevernever, so it takes place after Summer Knight. But could still have been before Death Masks
The prologue establishes that Harry was on the way to Edinburgh to report on his duel with Ortega. So definitely post Death Masks
And then there's this quote: "A low, feminine chuckle from nowhere echoed in response. It somehow combined smug satisfaction with an ‘I told you so’ quality. But I had no time to investigate. Twilight’s life hung in the balance." After Harry mentally cages the Nightmare, is a pretty likely a reference to Lash
Fridge Horror: In chapter 14 of False Masks, it's offhandedly mentioned that King Sombra (referred to as the last king of the Crystal Empire) went insane after trying and ultimately failing to duplicate the powers possessed by Obsidian the Undying. Let that sink in for a second. At the beginning of season three of the show, we actually saw some of the horrific dark magic that Sombra was capable of. If all of that was just a failed imitation of the real thing, then it means that as dangerous as Sombra was, Obsidian was much, MUCH worse.
Fridge Logic: Harry was purposely avoiding Michael during the periods the stories are set. So why did Harry call him for a ride to where he needs to be?
I'm not one hundred percent sure here, but I don't think Harry started totally avoiding Michael until after Dead Beat, when he became aware of Lasciel's shadow inside his head. Before that he was reluctant to speak with Michael, but I think that's when Harry really started avoiding him.