I've done this one!
They are cool murderous little jerks. I've used them myself on players a few times over the years.
They have the distinction of having killed a pc in my game!
Good job on the 5E write up. It's been quite some time since I used them, but I recall them being able to kill before their victim even know they are there. First time I used one, I gave the player a saving throw to wake before the assassin imp attacked. They made it and detected the imp forcing a morale check so it decided to escape rather than fight fair. I didnt think it was fair to just murder a player outright (which if ran as written should happen) but I was feeling generous that day.
