A Week Full of Daggers: Trouper's Knife

This one's a little dark, touches on potential body horror, gore, and body identity issues, which may make some folks uncomfortable.  You have been warned.  Read at your own risk.

Trouper's Blade

Description:  A small, scalpel-like blade with a grip made of bone bearing ominous stains.  Clearly more of a tool than a weapon.  Magical investigation will reveal an aura of transformative magic and a dark psychic taint of terrible pain.

Powers:  The blade can be used to alter a willing, restrained, or unconscious living humanoid creature's exterior form by slowly skinning it alive over the course of a ten minute long operation, with the new form being "carved" out of the old.  The wielder must keep the desired final form fixed in their mind for the duration of the process and should avoid distractions at all costs at risk of death or mutilation from an incomplete ritual.  The degree of alteration involved is considerable, allowing changes in height (up to a foot for an adult human), weight, skin, eye, & hair color, facial features, and distinguishing marks, but the subject must remain humanoid and organs or limbs can't be added.  Interestingly, the subject's teeth cannot be altered, only repositioned slightly, which may limit the degree that the jawbone can be changed as a consequence.  The transformation leaves the subject physically healthy but somewhat tired, and if they were conscious during the process they may well be mentally traumatized by the pain involved.   

Unlike similar magical transformations or illusions, the final form is permanent (barring another session under the knife) and can't be dispelled or detected as magical.  For safety, a subject must wait at least a month between future rituals, and attempting one before that should have significant risk of death (Fortitude save or save vs. death with a -1 penalty per week you've rushed things in D&D terms).

The Trouper's Blade itself can be used at will, and is always ready for a new subject.  Users may find the process of carving a new form into living flesh taxing however, and it is inadvisable to attempt to preform the ritual more than once per day.  Again, consider checks to retain concentration if pushing the envelope.  

Possible Origins:  Most scholars credit the invention of these nasty little things to a former spymaster of a whatever fallen kingdom/empire/civilization haunts your campaign, where they were used to make nearly impenetrable disguises for agents, assassins, and spies.  In modern times they still serve a similar purpose, but have also been adopted by the most fanatical devotees of the thespian arts as the ultimate way to truly step into a role on stage.  While few actors would agree to be literally reshaped for a part, there are always exceptions and for some the lust for fame makes anything worthwhile.   

Complications:  Using the thing requires you to spend at least ten minutes skinning a person alive, often someone you know and who requested you perform the task.  The process is intensely painful (although the blade's magic ensures survival if the ritual isn't interrupted), extremely grisly, and messy.  The spilled blood (so much blood) and former skin of the subject don't magically vanish and will need to be disposed of.  Losing your focus during the process will leave the victim disfigured or dead, and it's very hard to keep your focus if (say) an angry mob breaks in and demands to know what you're doing.  Privacy and plenty of cleaning supplies are a must.

Most voluntary subjects of the blade drug themselves into oblivion before the ritual for obvious reasons.  Attempts to bull your way through a transformation while conscious (perhaps by a  dedicated masochist or someone who's desperate and in a rush) may result in so much screaming and thrashing that the user loses focus with tragic results.  Involuntary victims must be thoroughly restrained or deeply unconscious, the latter being easier on the user's ears.

Design Commentary:  Rather obviously, before you use this thing in your game, make sure your table is okay with gory descriptions and body horror.  It might also raise personal identity and body dysmorphia concerns for some folks.  It can definitely be a button-puncher, so be careful.  Not worth losing a player (or a whole campaign) to a single magic item.

If you happen to running an urban horror game with it (eg World of Darkness) note that it only works on living humanoids, so any vampires or other undead will have to find a different way to change their form.  Myself, I used it in a bleak, grim & gritty fantasy espionage campaign, and it was by no means the worst thing the players ran in to.