"Well, that was quite the heist, eh?" Brynjolf asked, bumping fists with the thief next to him. The thief wore a mask, obscuring her face. She nodded, and Brynjolf took a swig from a bottle of Nord mead.
Recently, Birna had disappeared from Winterhold to go visit family. The thief had decided to ask Brynjolf to come up and raid the store. Brynjolf didn't need to know that she had killed Birna and thrown the body off of one of Winterhold's cliffs. "The Guild will be fed for the next month, warmed by mead. Nice fetch, lass. I'm heading out, feel free to search for any spare change while you're here."
Brynjolf walked casually out of the front door, not fearing the guards. The thief went upstairs and searched for loose septims in the drawers. She found some gold, but what intrigued her the most was a worn notebook. Without opening it, she dropped it silently in her bag. There'd be time to read it later.
For now, there was someone she needed to kill.
Birna's body was found a couple of days later by some hikers, the stab in her back carefully covered up by the assassin. After careful examination, the guards blamed the broken body on suicide and sent it to Falkreath.
Of course, the citizens were rattled. What scared them even more was the next body, found floating in the ocean near the shore. One of the guards that was most invested in the case of Birna's mysterious death. His clothes were discarded on the shore, as if he had gone swimming and drowned. The other guards were quick to dismiss it as a 'foolish youth underestimating the temperature', as their hands were full with the college and all.
It wasn't until the third death that the guards became worried. The body of Ranmir, brother of the late Birna. The assassination wasn't even hidden that time. Blood spilled from Ranmir's neck and pooled on the frozen ground.
The assassin was getting sloppy.