You get this for killing a certain someone and then finding a certain someone else at the sametime.
This is really the climax of the 'A Mysterious Murder' quest, I found this guide helpful:
http://divinity.wikia.com/wiki/Divinity:_Original_Sin_-_Ques...I also found this site to be a relatively indepth guide to the game; although this game allows lots of options so to fully experience it I wouldn't recommend following it to the letter.
http://psnprofiles.com/guide/3793-Divinity-Original-Sin-Enha...I would strongly advise being level 5, close to level 6 when you get near to the end of this quest. I would also not leave the north gate of the city before this point and advise going west first.
If you have a decent charm level, you can skip multiple encounters near the end of this quest.
Just in case their wiki goes down in the years to come before TA here is a paste:
*** Spoiler - click to reveal ***Walkthrough
After Orc Fight On The Beach, head towards Cyseal's gate to talk to Arhu and update your Journal.
Speak to Aureus at Legionnaire HQ for permission to investigate the scene of the crime.
Speak to Arhu upstairs for some more info about the night of the murder. He suggests you question Roberts at the Morgue.
Visit the scene of the crime. A Star Stone will activate and will either teleport you to The End of Time or unlock the Hall of Heroes depending on it being your 1st or 2nd stone. Pick up the letter in the chest to obtain or update The Councillor's Wife.
Visit Roberts in the Mortician's Morgue (180 XP) first or head straight to the graveyard and dig up the grave with the tombstone that say "Her Lies Jake". Open the coffin for 390 XP (180 Expl XP). You can now update Little Bo Bertia Lost Her Sheep.
Head to the Mortician's Morgue and save before you speak to Roberts. Ask him about Jake's body to initiate a game of RPS. Win the game to update the quest (230 XP, 150 Char XP). You can now speak to Aureus about having Roberts arrested for a choice between +1 Righteous or +1 Renegade. You can now complete Little Bo Bertia Lost Her Sheep as well. For an extra 100 exp, find Roberts' journal and read it before speaking to him. Roberts' journal is in the first floor room with the two corpses on the left as you enter. This is also how you can advance the quest if you lose the game of RPS; you can either sneak into the room (fairly easy to do when Roberts is upstairs) or kill Roberts, and then you'll have the run of the place.
(Optional) Ask Roberts about the gold and win another game of RPS (115 Char XP) to acquire it. You can either choose to keep this 300 gold (+1 Egotistical, -25 Attitude with Bertia) or return it to Bertia (+1 Altruistic, + 25 Attitude with Bertia). Either choice needs to be made in the dialogue with Bertia.
Now you must denounce Evelyn to proceed with this quest, so:
Complete The Councillor's Wife quest, and confront Esmeralda after completion.
(Optional) This is possible only if you have the Pet Pal Talent. Visit Legionnaire HQ and steal the Smelly Shoes from Aureus' cupboard. Visit Esmeralda's House and steal the Smelly Panties from her bedroom upstairs. Visit Thelyron's Healing House and steal Smelly Coat from the cupboard in the corner of the ward (where you were forced to choose which man's life to save). Visit Aureus' Office and steal the Smelly Scarf from the cupboard next to the bed. Return to the graveyard ask Murphy (Jake's dog) about Jake's body (100 XP) and have him smell all four items for 4 XP rewards. Murphy recognized Evelyn as a murder (360 XP).
Evelyn will now disappear. Return to the Ward room in Thelyron's Healing House and open Evelyn's Backpack for a key to her house. (Drag the backpack to your inventory if you want it). Speak with Thelyron about this and he will point to at her house (600 XP).
Head North West from here to Evelyn's House. Loot the Chest and Bookshelf in the South East corner for Evelyn's Diary and the Reveal Spell for 300XP. Read the Diary for 900XP.
Teleport to Cyseal North West and head South from where you confronted Dietmar. If you do not have this waypoint yet, you will have to complete Arhu's Failed Experiment and go through the West exit in the cave. You will also want to kill Dietmar to update this quest. Doing so will also allow you to update Cecil's Mighty Staff.
Use the Reveal Spell (1050 XP) when standing next to the torches South of Dietmar's location. Your companions will comment on the hidden lair when you are nearby.
Enter the hideout and kill the lone Cultist before he can return to the Orcs. He will have a key on him.
Either fight and kill the Orcs for 4 x 900 XP or win a game of RPS for 790 Charisma XP.
Unlock and go through the door and then through the next door immediately on your right. You can either ignore or kill (9000 XP*) the numerous Cultists before you go through the door if you want.
Head down the stairs to face and kill Evelyn (quest update: 1050 XP). Speak to Zombie Jake afterward to complete the quest, finding out that the White Witch is the one who killed Councillor Jake. If you already killed Evelyn in town, you will instead face her again as a zombie.