This is called 'Cult Master' and is unlocked when you 'defeat the cultists.'
It is the final mission in the DLC. You need to :
1. Drive to the Docks.
2. Protect the Police Boat. TIP: shoot the enemies on each boat, not the boat itself.
3. Eliminate the Cultists on the Barge.
4. Get into position
5. Eliminate the Cultists on the Barge. (second wave)
6. Find the Cultist Master
7. Arrest the suspect
Watch the cut screen and then 'pop'. Easy as that
I have included a YouTube video if you need further help. Credit goes to user DanQ8000.
Any questions / comments, let me know below.
Go get a pork bun :)