Gennadios the Phylakitai (Main Quest)
NOTE: Completing this quest sets a number of high-level bouty hunters on your trail, continually dogging your footsteps. If you still plan to idle around doing side quests a while, you may want to put this one off 'til later.
This quest is obtained immediately after Part I of the Aya Quest. Gennadios is the one hunting her- now you hunt him. Gennadios can appear in several location throughout the city, but the rules of engagement remain fairly constant- he'll be trailed by two body guards, and killing him is less difficult than getting some uninterrupted time with his corpse to confirm his death. To this end,it's best to wait until he's in a relatively hard-to-swarm area (alley, small room, parapet), then snipe him.
His guards are worth sniping too, if you have arrows left, but chances are that they'll get lost simply trying to find you. Just make sure they don't sneak up behind you as you make your way to Gennadrios' body and confirm the kill.
As with the masks, you'll briefly enter the afterlife, where Gennadrios isn't too happy with you.
Completing this conversation ends the mission, but there is a new wrinkle.
The elite Phylakes are now on your trail- wherever you go they'll be continually closing in on you (though fast travel or even simple evasion will throw them). They'll also respond with the reinforcements if you set off alarms in a restricted area, so consider this incentive to be even sneakier than usual.