The intel you gathered in the Nuit Blanche gives you three targets to investigate. You can hit them in any order, but this guide will tackle them by ease of access and completion, starting with the Wells Park Middle School.
Drive out to the school. When you get close, an investigation zone will pop up around it. The front door's blocked, but if you bear left around the building, you'll find a little alley with a ladder (there are an abundance of ladders in this DLC).
Get to the roof, then drop down through the hole to trigger the investigation (and begin breathing in lungfuls and lungfuls of an unknown hallucinogen).
Follow your hallucinations to a shrine that wouldn't look out of place in Karnaca.
Examine it, photograph it, then pull out the chalice in its gut, and rotate that for one last photo.
And that's it- school's out forever
There is an optional note here you can read, but after that, head up the ramp to the open window.
Exit school grounds to complete the mission, and set course for Welty Storage. You may want to drive something bulletproof, as you can expect to face multiple Retaliation squads on your journey.
The first squad just has cars, the second will roll in with cars and garbage truck. As usual, if you're going to make a stand, do so from cover. Once you've deaded the death cultists, proceed to the warehouse. As before, you'll use a ladder to access the building- this one is over a giant tank, to the south of the main entrance.
Slip inside and activate your flashlight. Follow the path of glowing paint- there are a few suspense moments here, but you won't be attacked just yet. You'll have to push open a few doors and climb a float, but eventually the path will lead to another ladder.
Climb up and proceed along the path until you fall through to the floor below. Pick yourself up and get ready for for fight- minimap and Tactical Vision both will warn you about group of cultists up ahead.
It's too late to save the sacrificial victim, but you can at least slay the sacrificers. Once they're properly dead, examine the scene.
There are two optional notes on the table behind the corpse. On the corpse itself, examine the face, wrists, and chest wound to gather all necessary clues. Exit through the far door. This path will lead you back to the door at the top of the ladder. It will also give you a vantage on some incoming baddies.
Go back the ladder and the light go out- completely out. You can't turn on your flash light, so use the minimap to watch for incoming hostiles, and direct your fire accordingly. One you kill one of them, the lights will pop back on.
The corners here make it a whistle-killer's paradise- you can reach the maindoor without firing a shot, if you like. Just keep an eye on the minimap so as not to trap yourself.
The main doors offer fewer opportunities for stealth. There are a ton of killers hiding behind cars out there. Best bet is blow up the cars (with either explosives or bullets), then gun down any panicked survivors. Either way, leaving the environs ends the mission.
The last stop is the Harless Mansion. You can expect another Retaliation team to ambush you en route. There's also a Retaliation team in permanent ambush just past the front gate of the mansion- they'll roll a car down on you. Still, be careful about falling back too much- firing in from the street will alarm witnesses.
Once you've disposed of the squad, climb the mansion stairs and bear left, up some weirdly tilting collapsed roof.
Drop down through the roofhole (as per usual) and scope out the mansion. When the first hallucination ends, inspect and photograph the General's old uniform.
You can also read an optional note on the table here, and listen to an audiolog before heading downstairs.
In a bit of dirty pool, you'll suddenly go from being unable to draw your weapon to being ambushed by maniacs. Give them the ol' lead nap before exploring the rest of the level (note that, as above, you can deploy the blacklight here, but it doesn't seem to reveal anything). Head to the western end of the wing for another drop hole.
The shrine is in the basement, along with all that remains of the General.
Inspect him, and photograph his book before turning to the shrine. You can activate the audiolog as well, but it's mostly madness. Examine and photograph the shrine itself to get your new marching orders.
Exit through the barrel-flanked door and up the ramp to leave the mansion.
With all three sites examined, return to Father James in the parishioner's apartment to discuss your findings with him and Anna.
Anna's horrified about the massive amount of hallucinogen you've ingested (and frankly, you should be too), but regardless, the trail leads to the old opera house.