Dark Squared Bishop
The Lincolnscire story arc ends with the assassination of Bishop Herefrith. The bishop is hiding in a chapel inside the fort at Anecastre. The three doors to the chapel are barred from the inside. That leaves three other ways for Eivor to enter the chapel: break a second story stained glass window on either side of the near end (look for exterior ladders to wooden platforms), or take the secret underground tunnel from the outbuilding past the far end. I chose the secret entrance to demonstrate how to find it and also because it puts Eivor closer to the target.
Most prime targets in AC games are on high alert and take one of three actions if you're exposed: fight, flee, or cower. Bishop Herefrith does none of the above. He just goes about his business. The ten guards inside the chapel with him are in informed mode and ready to fight.
I waited until the bishop steps on the "dark square" (chess analogy) beneath the candelabra and then dropped it on him. You still have to confirm the kill by holding 
for a second. After the cutscene there's no need for an escape. All the guards are gone and the assault on Anecastre is complete.