Priest's Debt
This quest is given by Eliana Passmore as a part of main questline. She can be found in the apartment A1 of Kolhola A building and officially she is a photographer. It is unknown what exactly relationship she has with Mr. Passmore, but given the shared last name and references to him as "old man", it seems entirely likely for her to be his daughter or possibly niece.
After exploring the Bergmann Maintenance Tunnels and turning in the results to Passmore as requested, he will refer the player to Eliana, implying that she is the one with the technical skills to produce forged passports and that the reason why he is not offering them anymore is due to falling out between them. Eliana`s banter seems to support that theory, expressing her lack of desire to be embroidered in the "old man" schemes more then she already was.
Eliana promises to discuss the question of passport if the player performs a little favor for her first. The favor consists of visiting the Deekula B and checking on the priest who lives there. Father Tobias can be found in the bed in the apartment under the blanket. His eyes are covered with blindfold, but the rest of visible face is covered in green and red mushrooms. While he visibly still breathes, he is completely unresponsive to the greetings.
At this point, leaving to report the findings to Eliana is an option and she will consider the chore to be sufficiently addressed, if so. However, notes found around the apartment strongly suggest that his neighbor from B2 had something to do with his current condition. The front door to B2 is locked and can not be lockpicked, however there exists a secret passage from B1 to B2. It can be found behind the bookshelf across the room from priest on the bed, along with big red button to make it open. However, for the button to become functional, one needs to obtain and bring Industrial Fuse and plug it into the slot near the apartment door.
Once the B2 is accessed, player can locate OSRat device and Black Amulet. It is an experimental device likely developed by the mysterious dweller of B2 that allows one to briefly visit Other Side. Black Amulet MUST be picked up in order for OSRat to work. The egress is only possible in specific places, most commonly next to the mushroom-riddled corpses. Or mushroom-riddled people about to become corpses. Using the device next to the priest (hold down the primary to charge it up and keep in mind that it will eat almost half of it's charge in one successful transition) allows to actually talk to his shadow sitting on the bed next to the body. Priest is being rather obtuse, however, and mostly speaks in riddles. The only certain thing he claims is that use of OSRat makes people enter the state they should not be in.
Reporting the results of this discussion to Eliana ends the quest for good. OSRat may be kept for personal use afterwards.