You'll actually most likely get this accidentally just going around the map, which happened to me. The shadowman is that creepy glowing purple old man who is talking to you throughout the map. You can see what he looks like as soon as you spawn in, by looking up towards junction he is standing on the catwalk. He has a multitude of location and is pretty obvious to spot, but here's where I have seen him, it seems like the first time you enter any area he is in, or complete a ritual, he will be there, so you dont need to continue leaving and coming back, anyway, here is where I have spotted him, you only need 5 of these locations.
-Catwalk above where you spawn.
-In the Footlight district, in the Burlesque, before you enter (where the ritual is done)
-In the Footlight district, on top of a shop (when the ritual is done
-In the Canal district, on the second floor in the Ruby Rabbit (where the ritual is done)
-In the Waterfront district, in the boxing ring before you enter (where the ritual is done)
- In the Waterfront district, on top of a boat just outside the boxing ring (when you finish the ritual)
-After completing all rituals and entering the rift and completing the ritual to unlock PAP he will be standing in the center of this room (not sure if this one counts)
If you go for the "beggining of the end" achievement on this map solo, you will almost certainly get this, as he likes to hang around the ritual sghts necessary for the achievement. If you are with a group make sur eyou stick together doing the rituals so nobody misses him.