Surprise, everyone! I finally got to playing Black Flags.
Early Death
Shortly before the assassination you tail the target on an eavesdropping mission. Right before the scene at the end you have a chance of killing both Prins and Torres (who I believe to also be a target in the game, although I haven't gotten that far yet) in a double air assassination.
It's a bit tricky since there are a couple of brutes right there, plus you don't want to do anything that would interrupt the conversation or the scene might start before the end of the conversation which would make predicting the time the scene starts impossible. Sleep darts help in keeping them at bay, but don't use beserk darts since that would draw their attention (in the video I used a beserk dart to lower a brute's health to kill him with one shot of the pistol).
It seems Ubisoft tried to make it impossible for an Early Death because shortly before the scene the targets are invincible to attacks, although there aren't for a split second before the scene starts (during that split second they change from yellow to red in eagle vision).
You don't actually see the kill, but you hear the sound of the blades. further proof is the glitched scene!
Completely Stealthy Pacifist Run
The goal is to be as stealthy as possible, the closest thing to a mysterious death. This means no yellow or red social status indicators. Start off by entering the place from the right. Sometimes I've gotten seen; it might have been because I wasn't fast enough or there might be some luck involved. After climbing the mansion stay to the left of the archer. There are two guards patrolling the backyard area. Move when their backs are turned and you won't have any trouble.
Now for the assassination. I chose a low profile fist kill because it was non lethal and matched the "mysterious" description. I may not be an expert on strangling, but I'd be willing to bet it can be done without leaving any marks.
"Employee of the Month"
For the fun of it I wanted to try killing the target with a beserked guard. the closest guard patrolling the backyard area is the best option, but it will take luring to get him to directly go after the target. When he comes from one side of the patrol he temporarily stops by the alarm bell. From the stalking zone you are close enough to attract him with a whistle. I took out the other guard by the back of the house because the beserked guard might go after him instead, depending on whether he or the target is closer.
When in combat with the poisoned guard the target gets in a battle stance but makes no attempt to attack (also if you poison the target to fight you he won't attack either.) Now for a surprise. By doing this I managed to complete an Abstergo challenge and got the Employee of the Month achievement for completing twenty five Abstergo challenges.