Slasher Smiles is yet another Eclipse of Equestria character. She is an assassin moon pony (or batpony) in employ of the Followers of Nightmare. She is quite deranged, which is practically a requirement for the Followers, seeing how they seek to end all life in Equestria and usher in eternal night (these two goals can be completed in any order, really).
Unlike many other characters, Slasher uses a lot of Old World arcane tools, rather than the hi-tech tools that were developed in the final years of Equestria. In addition to vibroblades on her forehooves and impact spikes on her hind legs, she also possesses a very rare magic talisman that allows her to turn invisible. Rather than wearing it around her neck, or wearing it at all, she has this talisman implanted into her chest, powered by her own life force. Its drain is not too high, and she can remain invisible for days on end without harm to her physical health, but it does not do much good to her mental health.

Slasher is actually based on a different character by the same name: http://www.furaffinity.net/view/5723165/ . She is indeed heavily inspired by the Keatonpony', but it is not quite the same character.

MLP:FiM © Hasbro
Eclipse Phase © Posthuman Studios
Slasher Smiles © me