Can someone pinpoint for me the exact location of the kingslayers' lair? There is a bug present that causes the quest marker to jump all over the map so you can't rely on the map to tell you anything.

Head towards the house on the cliff. (Where you did the Little Sisters quest). As you leave the river and head into the gullies, the path branches. The right fork goes to the House on the Cliff. Take the left fork. The path wanders around for a bit and then has another junction. Going straight ahead would take you into the mist, turn right instead to go to the assassins's lair.