These attacks can occur only in places considered to be habitations, and these locations must have both a location center marker and a location edge marker.
Therefore, the attacks can only occur at: Narzulbur, Dushnihk Yal, Whistling Mine, the Solitude Sawmill, Snow-Shod Farm, Sarethi Farm, Salvius Farm, Pelagia Farm, Half-Moon Mill, Merryfair Farm, Loreius Farm, Left Hand Mine, Kolskegger Mine, Katla's Farm, Hollyfrost Farm, Heartwood Mill, Chillfurrow Farm, Brandy-Mug Farm, Battle-Born Farm, Anga's Mill, Old Hroldan, Karthwasten, Stonehills, Nightgate Inn, Kynesgrove, Shor's Stone, Ivarstead, Rorikstead, Riverwood, and the cities of Solitude, Markarth, Riften, Windhelm, Whiterun, Morthal, Winterhold, Dawnstar, and Falkreath.
They occur depending on a random chance (5% if it is nighttime, or 25% if an attack occurred during an eclipse caused by Auriel's Bow) or if it has been long enough since the last attack (twenty in-game days). Multiple attacks cannot occur on the same game day.
There are two caveats to the above: sometimes, for unknown reasons, the game can trigger two attacks simultaneously, causing both of the attack types listed below to occur. In addition, if you are extremely unlucky, you might trigger and attack before midnight and one after midnight, since the game day changes then.
The following attack types can occur if it is nighttime:
-One vampire boss with two bandit thralls (one ranged, one melee)
-One vampire boss and two Death Hounds
The following attacks can occur during an eclipse:
The two attacks above, plus:
-A gargoyle (player must be level 20+)
-A vampire boss and a gargoyle (player must be level 20+)
-Three Death Hounds
-A vampire boss, a Death Hound, and a gargoyle (player must be level 20+)
Nighttime attacks cease once "Kindred Judgment" has been completed, but eclipse attacks can still occur.
You will always know that an attack is happening, even if you cannot see it, because guards will always say one of the following:
"These vampires are becoming a real menace."
"Where're these vampires coming from? We need to wipe them out before things get worse."
"Dragons breathing fire in the sky. Vampires brazenly attacking people on the street. It's the end of the world I tell you."
"They say if a vampire so much as scratches you, you'll turn into one. That better not be true."
"We need to do something about these vampire attacks."
They will say one of these lines once per attack.
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The robe-clad disguised traveler has a 25% chance of appearing in habitations (including the above listed, but a few other locations as well). He only appears between the hours of 10PM and 4 AM, and the player must have joined the Dawnguard.
He will attack if the player spends three seconds in too close a proximity to him (300 units, which is about the distance from the doorway of the player's room at the College of Winterhold to the center of the magical well in the center of the room), or if the player attempts to speak to him. He is as strong as a normal vampire boss.
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Feel free to ask questions or for clarification.