This little fiendish dagger is something my rogue loves. When you are in front of an enemy the only thing visible is the handle of a dagger. It appears completely harmless without an arcana check on it. But when the dagger is behind someone's back a shiny red blade appears from the handle. When put in someone's back side on top of the original damage(1d4+2) it causes them to bleed continuously for a set time (I set it at 1d4 per turn for a minute in combat) If the enemy dies by the stabbing or by bleeding out a red insignia that the killer choses is magically burned on the back of the neck as a calling card. You can scale damage however you like for your players but I wanted to make a dagger that a rogue would still feel good using at higher levels. This definitely helps my rogue play into their character.
I really like the flavor of this item. It really only works while being a rogue, or at least having rogue like tendencies. To help with scaling, and assuming this is 5e, you could allow for stacking bleed damage from multiple hits in a single round. The other idea that popped into my head was to have this initially presented in a crime scene fashion where the mark is on a victim and no one can figure out what kind of blade was used to kill them. Then the BBEG has the weapon, but people can't find the right evidence because it just looks like a handle.