Eriah rejoined Brynjolf at Mercer's desk. As she was instructed, she touched bases with Vex to get past the thug Mercer hired to watch his house. The man owed a debt to Maven so a quick trip to see the woman in question and a dip in Lake Honrich later, Eriah returned a quill to Maven who then gave her a writ to free Vald of his debt. The man walked away a free man and she took care of the rest. As Brynjolf said, hired mercenaries were crawling about the place and she handled them all while scouring the home for anything. She located a secret passage behind a wardrobe with a false back and, dodging all the traps found within, she came into what looked like an office. Mercer had left the plans he had stolen from the Guild lying around so she grabbed what she needed and found a drop hole that led to the Ratway. From there, she made her way back to the Cistern.
"We've scoured the town and I've spoken to every contact we have left. No sign of Mercer. Any luck on your end?" Brynjolf asked. "He wasn't there, but I found these plans," the Dragonborn responded, handing them over. "Shor's beard! He's going after the Eyes of the Falmer? That was Gallus's pet project. If he gets his hands on them, you can be certain he'll be gone for good and set up for life," the Guild's number two gasped as he read them over. "We can't let him get away," Eriah said. "Agreed. He's taken everything the Guild has left, and to go after one of the last greatest heists is just an insult. I've spoken to Karliah, and made amends for how the Guild's treated her. Now she wishes to speak with both of us. Quickly, we have no time to lose," the man said, leading her to where Karliah was standing in the center of the Cistern.
"Brynjolf, the time has come to decide Mercer's fate. Until a new Guild Master is chosen, the decision falls to you," the Dunmer said after nodding a greeting to Eriah. The Dragonborn steeled her shoulders. There was no time to lose. "Aye, lass... and I've come to a decision. Mercer Frey tried to kill both of you, he betrayed the Guild, murdered Gallus and made us question our future. He needs to die," the man in question said, gravely. "We have to be very careful, Brynjolf. Mercer is a Nightingale, an Agent of Nocturnal," he was warned. That had been the third or fourth time Eriah had heard that term. She didn't know the intricacies of it but she surmised from listening to the talk between Karliah and Mercer that the Nightingales had some sort of power from Nocturnal, Daedric Prince of the night, stealth, and luck. The Dragonborn hoped she didn't have to get involved with her. Molag Bal had been a terrifying encounter and Clavicus Vile had been a pain in the ass. Then there was Hermaeus Mora who was even more of a pain in the ass than Vile and he was the Prince who hurt Eriah the most. Nothing good ever came from dealing with Daedra.
"Then it's all true... everything I heard in the stories. The Nightingales, their allegiance to Nocturnal and the Twilight Sepulcher," Brynjolf was saying. Eriah came out of her musings at his voice. "Yes. That's why we need to prepare ourselves and meet Mercer on equal footing. Just outside of Riften, beyond the Southeast Gate is a small path cut up the mountainside. At the end of that path is a clearing and an old standing stone. I'd ask you both meet me there," Karliah said. She walked off as Brynjolf turned to Eriah. The Nord woman sighed and rubbed her eyes. "Septim for your thoughts, lass?" he asked. "Seems I'm about to meet another Daedric Prince. Not what I was hoping for. Still, there's no time to find another way. Let's just go," she said, following in the Dunmer's footsteps. Brynjolf watched her before doing the same.
Outside Riften, Brynjolf and Eriah found Karliah where she instructed them to be. She nodded to them and led them inside a cave. They emerged into a hall and her companions looked around in awe. The hall was neglected and appeared to have been in rough shape for quite a long time. "So, this is Nightingale Hall. I heard about this place when I joined the Guild, but I never believed it existed," Brynjolf said. "The assumption that the Nightingales were just a myth was seeded within the Guild on purpose. It helped avert attention from our true nature. What's wrong, Brynjolf? I can almost hear your brow furrowing," Karliah said as she led the two down a hallway. "I'm trying to understand why I'm here, lass. I'm no priest, and I'm certainly not religious. Why pick me?" the man asked. "This isn't about religion, Brynjolf... it's business. This is Nightingale Hall. You're the first of the uninitiated to set foot inside in over a century. Now, if you'll both proceed to the armory to don your Nightingale Armor, we can begin the Oath," the Dunmer responded.
The three found the armory, really a set of three stones with what had to be the symbol of the Nightingales. While Karliah and Brynjolf approached the stones, Eriah hung back. "What's wrong?" the Dunmer asked when she noticed the Dragonborn was hesitating. "It's nothing," she responded. They weren't buying it but said nothing. Eriah approached the center stone and held her hand over it, her friends doing the same. A set of light armor colored black appeared, complete with a cape and cowl. Heading off to a room to switch armor sets, Eriah groaned inwardly when she noticed how skintight it was. She felt exposed but she sucked it up and joined her friends outside, leaving her Blades armor in the room she changed in.
"I do not like how this fits," she grumbled. The other two just chuckled. "Okay, lass. We've got these getups on... now what?" Brynjolf asked, looking at Karliah. "Beyond this gate is the first step in becoming a Nightingale," the woman responded. That sent Eriah on high alert. "Woah there, lass. I appreciate the armor, but becoming a Nightingale? That was never discussed," the man responded, seemingly agreeing with her unease. "To hold any hope of defeating Mercer, we must have Nocturnal at our backs. If she's to accept you as one of her own, an arrangement must be struck," Karliah said back. "What sort of arrangement? I need to know the terms," came the response."The terms are quite simple, Brynjolf. Nocturnal will allow you to become a Nightingale and use your abilities for whatever you wish. And in return, both in life and in death, you must serve as a guardian of the Twilight Sepulcher," she replied. Brynjolf sighed in defeat. In the end, his need for revenge outweighed all else. "Aye, there's always a catch. But at this point, I suppose there isn't much to lose. If it means the end of Mercer Frey, you can count me in," he agreed.
The two then looked at the quiet Dragonborn. She was staring intently at the stone floor off to the side. "What's wrong?" Brynjolf asked, stepping towards her. Eriah looked at her friends. "I don't have much of a choice, do I? It's been my way to avoid contracting myself to Daedra," she replied. Karliah and Brynjolf looked at each other, understanding. "I'm sorry, Eriah. I'm afraid that if we're to meet Mercer on more equal footing, you'll have to become an Agent of Nocturnal to do so. I don't deny your power as the Dragonborn but it may not be enough. Mercer has the Skeleton Key, which makes him more dangerous than just his Nightingale status alone. This mission requires stealth of the highest order and because you're not a thief by trade, you need the most help of the three of us. Brynjolf and I could probably take him alone but I believe deep down it will take all three of us. We need you, Eriah," Karliah explained.
Brynjolf closed the distance to set a hand on her shoulder. "The lass has a point. Also, if you walk away now and if Karliah and I fail to take out Mercer, there's a chance he may come after you to tie up any loose ends before he disappears for good. I, for one, can't let that happen. You're the best thing that's happened to the Guild in a long time and you've become an important friend to me. Please," he said. Eriah looked down, warring with the decision before her. Bending the knee to Nocturnal meant that her afterlife was on the line. Eriah wanted Sovngarde after death and she didn't want to jeopardize that. But her friends had a point. Mercer had some powerful abilities that her power as Dragonborn probably couldn't overcome. As Brynjolf had said before, she was in too deep now and she herself had sworn to finish Mercer off for what he had done. She internally started making plans to hopefully alter her own terms if Nocturnal did speak to them. She was considered a reasonable Prince by many and thus might be willing to hear Eriah out. Not letting her friends in on her thoughts, she looked at them each in turn. "You're both right. I can't afford to walk away. Let's get this over with so we can finish the job," she said. The other two nodded grimly and they headed past the gate Karliah had pointed out.