The Source

The Source is a main quest in Act III of The Witcher, covering Vizima’s Trade Quarter and Temple Quarter. At the start of the act Triss Merigold tells Geralt she has detected a magical anomaly somewhere in the city, the kind that could draw even more monsters into Vizima, and she needs his help to pin it down.

Three magic sensors later, the anomaly turns out to be Alvin, the boy Geralt saved in Act I, whose Elder Blood could turn dangerous without proper training. Triss wants Alvin brought to her; Shani, who has little love for Triss, wants him brought to her instead. Before Geralt can settle it, Alvin is snatched from the hospital, and it falls to Geralt and Dandelion to get him back and then decide whose care he ends up in.

Walkthrough

Talk to Triss at her house at the start of Act III and offer your help; she gives you three magic sensors.
Place the sensors at the three reliefs: on the east side of the Trade Quarter next to the bank, in the Temple Quarter near the sewer entrance, and at the cemetery.
Return to Triss (at her house or the New Narakort inn). She traces the anomaly to Alvin at St. Lebioda’s hospital, asks you to bring him to her and hands you the Formula for De Vries’ Extract.
In the Temple Quarter, Shani asks you to bring Alvin to her instead.
At the hospital, Dandelion tells you Alvin was taken to a house in the non-human district and leads you there (you can also go straight to the house).
Kill the four assassins inside, decide whether Alvin goes to Shani or Triss, then deal with the second wave of assassins.
Tell both women what happened, starting with the one you chose, and finish by talking to Triss.

Quest Consequences & Choices

Alvin goes to Triss or Shani: the woman you chose can become Geralt’s lover; the other turns him down, and Triss does not take being passed over quietly.
Beyond that, the choice only changes small details in Act IV and the epilogue.