Not sure what you want the Staff to do, but the magic on the Emssary shouldn't be too tough- Sponsored Magic with a lightning/electricity and possibly "Turn myself into a goose to seduce ladies" theme.

I like it, personally I would also have some kind of marker to show outwardly that the sword was in use, say milky or completely black eyes or darkened veins.

Didn't think of that, but I like the idea. I'd probably go with veins, myself, as it's more of a physical power than a mental one.

Going beyond the inhuman powers might be a bit much IMHO. I've always felt though that demonic co pilot should constitute some kind of mental and social protection since you're not in control, so 'getting through' to someone who has been subverted by the sword is hard to bring back. I'd do this in the form of mental and social 'armor'

You may be right about post-Inhuman powers; on the other hand, this thing's a murderspirit backed up by seven centuries of killing things...

On the second point- sounds like a general change to Demonic Co-Pilot to me, though you could also model this as a Compel to the Aspect related to the Sword forcing them to resist any attempts with an appropriate skill, which would of course pick up the Demonic Co-Pilot skill bonus.

Are you interested in making the sword able to turn PCs into NPCs if they use it or fall to it?

Where appropriate, yes. That's the main reason for the powerup mechanic- if you grab enough of those powers to hit zero Refresh, NPCtime.

I'd only want that to happen as a direct player choice, though, anything else seems against the spirit of FATE.

Also, I may put up a few ideas on what the Sword is and why it's so powerful...

True- so for, say, a Loup-Garou with the Catch "Inherited silver weapons" and Mythic Recovery, I'd have "Arm cut off" as a Severe consequence, and "Arm cut off with an inherited silver weapon" as an Extreme consequence. I think Extreme consequences are extreme enough to justify that.

To be clear- I wouldn't be downgrading normal consequences because "this guy heals fast, so they matter less," as that devalues the healing power; Extreme ones are already an exception to several of the standard rules, as they modify an existing Aspect- thereby changing the core of the character- so I think they deserve special treatment.

Honestly? I wouldn't give that guy One Arm as an Extreme consequence. Consequences are a narrative abstraction, and for that guy, losing an arm doesn't hold the narrative significance it would for someone else- hence, it's not an Extreme Consequence.

(This is opinion.)

I'm playing a Warden that play has revealed to have a similarly short fuse. My suggestion to my GM was to change my Trouble to "Allergic to Bullshit."

I like that... And you can probably get away with Invoking that to spot when someone's trying to delay you, or to help intimidation, and stuff...

... I now wish to play a mercenary with the Aspect "There is no overkill- only "Open fire!" and "I need to reload."

This is something I put together essentially because I felt like it. Mechanically, its most obvious feature is taken from the Swords of the Cross- but background-wise, it's got a rather different feel...

I see this as a potential weapon for a PC (and I may play Last Scion of the d'Ortonnes Family sometime myself...), but it could just as easily be the spark for a plot in which the PCs have to find it and take it up temporarily to kill something very nasty, or to return it to safe hands before the bad guys can get hold of it and kill something that really shouldn't be killed.

The d'Ortonnes Sword (Item of Power) [-4 Refresh (base)]

This is a sword steeped in death. It was made to kill that which could not be killed- forged for Sir Joseph d'Ortonnes in the 13th Century to drive off a being of shadow and flame that was terrorising his estate. The sword was made by a robed stranger garbed as a monk, and Sir Joseph paid a terrible price for it- the stranger claimed a weapon suitable to destroy his foe required the sacrifice of seven innocent lives to give it the necessary power. Enough lives had been lost that he agreed, and seven of his peasants were taken by the monk, never to be seen again.

He returned with a sword- elegantly made from bright steel, and yet somehow... harsh. The light reflects cruelly from its edges, drawing attention to the sharp tip. It is quite clearly an instrument of death.

The Sword was passed down the family line from father to son- or daughter, in a few cases, when no sons were available- with a duty to care for it, use it to destroy those that prey on society- and to keep it from the hands of those who would misuse it.

For the d'Ortonnes Sword is a powerful tool, and easy to claim- it will accept a new wielder simply by a mingling of their blood and that of the previous wielder in a pit on the pommel. The family have been forced to fight off numerous assailants who sought it for their own ends, and even reclaimed it after a theft on more than one occasion.

The saddest of these, though, are when it must be taken from one of their own who has been seduced by the power they bear. It is a terrible weight, and one that they bear, for they cannot trust anyone else with it.

Effects:
Demonic Co-Pilot: Applies when the Sword is unsheathed and in use. The sword's spirit's Agenda is quite simple- it was created to kill, and it wants to kill. It doesn't really care what.
The End of Things: The bearer of the d'Ortonnes Sword may pay a fate point to ignore a single target's armour and supernatural protection for the duration of a single scene, as if it satisfied the Catch. [Equivalent to the power of the Swords of the Cross... though not nearly as benevolent.]
It's a Sword: Duh.
Unbreakable: As an Item of Power.
The Power Within: The spirit of the Sword has more to offer- at a price. Over seven centuries of bloodshed, it has amassed a great deal of power, and a willing wielder can allow some of this power in, making him incredibly strong and fast- the better to do the work of the Sword. The wielder may at any time gain Inhuman Strength and/or Speed, immediately modifying his Refresh total and paying two fate points as if using on-the-fly character creation. The wielder must also modify an Aspect to reflect the murderous impulses of the Sword- for example, Guardian Against Darkness might become Uncompromising Slayer of Darkness. The same process may be used to acquire Supernatural or even Mythic Strength and Speed- but the repeated changes to Aspects combined with the high Refresh cost means this is not for the faint-hearted...

That 12-shift Attack spell could be any number of things, and it's not unreasonable, even with something like a blast of fire, to go with a Taken Out condition like "Badly burned and unconscious." It might seem like playing the system a bit- but if someone seems to be rather less than appropriately careful than they should be, you can always Compel...