The video above is the The Witcher 3 Wild Hunt Possession Secondary Quest Walkthrough and shows how to complete Possession, the quest featured in The Witcher III: Wild Hunt.
The video walkthrough covers the following objectives:
Ask Udalryk where Cerys is.
Ask about Cerys in the village.
Talk to Bergthora.
Look for Cerys in the abandoned home using your Witcher Senses.
Find the sword named Brokvar.
Tell Cerys you found the sword.
Go to Udalruk with Cerys.
Place Brokvar near Aki’s bones.
Return to Udalryk.
Meet Cerys in Udalryk’s old home.
Search the house and find a way to drive off the hym.
Bring torches from the storage shed.
Place the torches around the abandoned home (4).
When you’re ready, talk to Udalryk.
Light the torches.
Kill the Hym.
Tell Udalryk it’s all over.