Slash
As far as Sigil is concerned, Slash is the assassin's assassin. Chant has it that nobody is safe if Slash is the one hired to put 'em in the dead-book. And so far, nobody's proved the chant wrong.
His origins are mysterious -- even more so than other Negatai. Slash stepped out of a known portal to the Negative Energy Plane approximately 15-20 years ago. Unlike most Negatai, Slash seemed to have a definite purpose when he emerged. Whereas most Genasi spend their first few weeks in Sigil adjusting to the Multiverse, Slash immediately begin searching for a portal to Toril, specifically the Kara-Tur region. When he found one, he stepped through and was not heard from again for a few years.
When he finally re-emerged, he again started seeking a portal -- this time for Zakhara, south of Faerun. Again, he stayed gone for a few years before re-emerging again to seek yet another portal.
This repeated itself every few years until one day Slash stepped back into Sigil and simply melted into the night. Seems he was through portal hopping. No one knows the true dark of his different trips around the Multiverse, but the chant is that Slash was studying under master assassins around the Multiverse, from the ninja of Kara-Tur to the hashashin of Zakhara to the former priests of Bhaal, and a variety of places in between. When he felt he'd learned enough in a particular area, he assassinated his instructor and left to find a new one.
Whatever the truth may be about his planewalking, there's no denying the fact that Slash is an unparalleled master of dealing death. He's responsible for over a hundred assassinations, and he's rumored to be responsible for over a hundred more.
Despite his noteriety, he always gives the law the laugh. The Harmonium routinely scrags him after a high-up gets greased (even if he didn't do it), and the Guvners routinely turn him loose due to lack of evidence. It's not that Slash kills witnesses; it's that he never lets himself be seen to begin with.
The most famous assassination rumored to be committed by Slash involved a tanar'ri high-up who was in a locked room. The room itself was in an extradimensional space, so the only access to the room was through the locked door (and past several dozen armed and extremely alert tanar'ri guards). The guards opened the door to report to the high-up and found him slumped dead over his books. No cause of death was ever found, and nobody ever discovered how the assassin entered the room.
Interestingly enough, Slash has a rigid (and somewhat surprising) code of honor. He will never accept a job to kill a child, nor will he kill someone who devotes large portions of their time and/or money to helping the poor. (And in fact, he's rumored to be responsible for a number of very large, very anonymous charitable contributions to organizations which help the poor and indigent.) He won't kill anyone unless he's paid, or unless he feels there's no other way to avoid being killed himself. This code of honor, combined with his high fees (it costs 1500gp, non-refundable, just to get an appointment with him to discuss an assassination job -- and that's in addition to whatever fee he charges for the assassination itself), is enough to thankfully keep the body count from dramatically rising in Sigil.
Slash is a bald Negatai of average build. Like all Negatai, his skin is so dark as to slightly absorb light. His only distinguishing feature is a scar he received from an unknown source. The scar starts above his left eye and travels diagonally down his face, across his nose, and ends just above his jaw on the right side of his face. For some as-yet-unkown reason, the scar itself is white.