Aethia, by Malckeor
Zero-ish spoiler summary: Three ponies search for treasure in the bloody homeland of the griffons, far from Equestria's peace and (relative) security. Unfortunately for them, there be dragons...
A few thoughts: I had the chance to read the beginning of this a while back, and it's improved by leaps and bounds since then. This story definitely earns its dark/gore tags, working its way pretty quickly through a bloody gambit from "dark humor" to "violent drama." Normally, that's not my cup of tea, and indeed I did find this a bit outside of my tastes, but Malckeor also works a lot of draconic racial tidbits into the story which I enjoyed, and has a knack for voicing; despite a significant all-OC cast, character dialogue was well-differentiated throughout, and exchanges between ponies and griffons were regular highlights of the fic. Focus is a bit of an issue; although one character's backstory, told in spurts through flashback, ties everything together, the story does seem to flit between emphasizing that pony's journey, and the broader adventure-cum-survival-tale which includes but doesn't necessarily focus on her.
Recommendation: Readers looking for something on the more violent end of the fanfic spectrum which isn't just edgy for the sake of edginess should definitely give this a look, as should fans of draconic race-building. Readers looking for something even slightly show-tone will want to steer away, however.
Iron Hearts Book 2 - Ferrous Dominus, by SFAccountant
Zero-ish spoiler summary: The second book in the Iron Hearts series, wherein the girls (with varying degrees of reluctance) travel to the Chaos legions' base on a mission to get the humans to leave Equestria. Of course, asking a bunch of murderous psychotics with unbelievable destructive capabilities to get lost isn't the sort of quest that comes without complications, and that's without even mentioning the Tau...
A few thoughts: I reviewed the first book in the story here; it being a necessary prelude to this story, I'm basing my review upon having read that story, and the assumption that anyone considering Ferrous Dominus will have read it as well. I'm also going to take my comments on that story as a given, as usual, and focus on things that are better, worse, or just different between that story and this. Anyway, this story starts to move away from the humor-via-ethical-contrasts (there's still quite a bit, but the characters do have to adapt eventually) and builds in a bit more scheming, plotting, and of course, warfare. The latter is still excellently written, even if some of the 40k-specific stuff is lost on me, but I found myself disappointed by the grand plans, at least as regarded two of the major players: Celestia and Serith (a human psyker (mage)). After the end of book one, it seemed that Celestia had some inkling of what was coming, and that this could develop into a major plot point. Here, her role is entirely reactionary and ineffectual, which made a disappointing contrast to expectations (and on the subject of princesses: although her sister has a more interesting role, I have to note that Luna's Ye Olde English is pretty awful). Serith, meanwhile, makes an interesting antagonist at first, but his motives remain opaque right through to the end. Moreover, it seems not to be a case of "to be revealed," but rather that he doesn't have any definite motivations which the fic plans to expand on. Hopefully I'm wrong on that front, but trying to figure out what he wants or values from the first two books alone is an exercise in futility.
Recommendation: If you haven't read book 1, then this is definitely not the place to start. If you read book 1 and enjoyed it, though, this follows immediately through on the end of that story, and despite my complaints, I found it to be a slightly more serious but still very darkly-humored action romp.