1. A pen for the finest of copperplate scripts, for the sort of swirling, twirly writing that spills out of spell scrolls or crawls across the ransom notes of mad wizards. To get this effect, one needs a long, flexible nib, sharpened at the end to a point capable not only of ripping through the paper if incorrectly used, but also of ripping through the fabric of the universe into some other, inkier space. Obviously you must be careful when handling such a pen. But perhaps more careful when handling things it has written, and in telling an inkblot from a hole into the void.
2. They say the pen is mightier than the sword. But why choose? A true literary assassin should have a pen that is also a sword, allowing you to leave a pithy note on the corpse of your duelling opponent like the loveable psychopath that you are.
3. The modern literary assassin, of course, has a pen that is also a gun. More specifically, if you are going to Bond it up with the literariest, you will need the sort of gun that shoots precision wordballs, primed such that their inky centres explode in mid-air through a minutely-lettered shell and print the nearby surroundings with the text of your choice. With such a weapon one might stun the right kind of burglar. Perhaps by printing their getaway van with a cheeky bit of Burroughs, or by leaving verses imprinted on the inside of their heart.
4. Ordinary pens lounging about undercover, hiding their mysterious histories by replacing their spun-silver hides with biro shells and their ancient nibs with blotchy rollerballs and their holy clockwork with springs and shavings. You may have one of these lying around and never know it. Save perhaps that one day it might forget itself and spurt out forgotten things in gold and sea-turquoise.
5. After the singularity, of course, we will have artificial-intelligence Shakespeares with ten thousand finger-pens. The Consciousness will set them loose, hundreds of the things, made in some humanless factory beneath the Kamchatka peninsula. They will crawl the forests of the world in search of trees to eat and shitting out reams of paper on which they write astonishing verse. But they will write on human skin if they have to. The peoples of that time will sometimes gang up to slay them, the pens being held to be tokens of good luck.