Bon Bon's voice is funny, and that's about all that anypony who meets her notices, as her marefriend Lyra soon steals the spotlight with her explosive personality, strange obsessions, and curious beliefs. And why not? Bon Bon is, after all, a perfectly ordinary, boring pony in every way but two: her voice, and the carefully kept secret that she isn't really a pony.
I'm a big fan of Changelings in My Little Pony fanfiction and this particular story gives a unique and incredibly well-realized interpretation of them. Dromicosuchus' Changelings are fae creatures in the classic Medieval European sense, dwelling halfway between the "real world" and a strange alternate universe of incomprehensible magic and terrible monsters. Many other fae creatures and details from European mythology are transplanted whole-cloth into this setting, most notably Kelpies (the secondary character Aldrovanda is a Kelpie and is both horrible and hilarious - a great combination).
The protagonist is Bon-Bon and it's no spoiler to say that she's actually a Changeling. However, unlike other Changelings, she has developed a genuine love for a pony rather than the usual sham Changelings use to feed on them. The story opens with her discovery of the impending invasion of Canterlot being planned by Queen Chrysalis, and as it progresses it weaves itself perfectly into the background of the actual events that take place during the two-parter episode "A Canterlot Wedding." As such it's always clear how the larger-scale plot will ultimately end; the invasion will fail. The journey is the important thing, though, and there's still plenty of room within that broad framework for edge-of-the-seat tension and uncertainty.
Fans of the White Wolf style of roleplaying setting may particularly enjoy this story, set as it is in a world with a "masquerade" similar to how many of those go. Bon-Bon is the viewpoint character, though, and she knows all about how the world really works so the reader should rarely feel lost. The setting has a well-realized feel to it.
I would like to say that this is not merely one of the best Pony Fics I have ever read but one of the best Novellas I have ever read. In it the Author weaves the rich folklore of the Celts, implementing the Faeries of old as eldritch abominations lurking in a venomous Otherworld rife with necromantic horror and Lovecraftian fear. Much of the descriptive prose is beautiful or horrifying, often a mix of both, and epitomizes the standard set by "Nyarlathotep;" poetic and descriptive while remaining concise. Now, as to the story itself I have to say it's quite entertaining. It is, for lack of a better word, an adventure story. It could technically be considered a romance, I. E. the thing driving the protagonist forward being that the love of Her life is in danger, though I'm not quite comfortable describing it as such. Likewise one of the particularly enjoyable parts of the story is the sarcastic back-and-forth between the protagonist, Bon Bon, and an unwanted travelling companion which follows Her on Her quest for much of the story. Their dialogue together is positively hilarious. Likewise there's some very good characterization in this story, and it contains one of My all-time favorite fictional characters. I won't go into detail, but He appears about halfway through and makes quite the entrance. Anyway, I would suggest anyone with a love for adventure stories, a fondness for folklore, leanings towards the Lovecraftian, or simply a familiarity with Pony to read this story.
As a side note, this story inspired My love for Celtic folklore. Before reading it the only myth-cycle I particularly cared for was the Norse, but Mendacity changed that. I loved it so much and became so interested in Celtic mythology that I, with much help from Dromicosuchus Himself, created the annotated edition of the story providing information on the real-world equivalents of the various mythological creatures mentioned in the story and other miscellaneous facts. This annotated edition has since been added here and can be found above.