Completing the Ritual
After that happens though, you'll be able to talk to Dethmold and Henselt to finish the preparations to break the curse. When you're ready, go to the execution site again and confirm that you're ready. I strongly suggest that you get some specter oil ready though, and potentially take a Swallow potion, then save.
In order to start the ritual, you'll need to draw out the weird goat skull on the ground, based off of the image that Dethmold gave you.
For this, you need to call out to Henselt and give him direction. He starts at the mushroom, so send him to the petrified bread, then the candles, the skull, the tree, the raven, the sour milk and then back to the mushrooms.
Once that's ready, you can go up to the circle and use Igni on the wagon wheel to kick things off. This is where things get bad really fast. Wraiths will continue to come into the circle, until you've killed about 10 of them. Specter oil is practically vital, and you'll probably want to keep Quen up at all times. For the most part, roll around and then hit them quickly with a series of fast strikes. Note that you'll want to stay in the circle, or you'll anger the wraiths outside and start to bleed off health.
If you're having trouble with the fight, you can use a bit of a cheesy tactics. The wraiths still set off all traps. Snare traps work surprisingly well against them, and if you need a break, you can easily cast Quen and use the cover to roll around and set traps that you can then lead the wraiths through to split them back up.
Play very cautiously and know when to back off. If the wraiths can start to chain up combos against you, you will be in a world of hurt. It's an endurance fight, so it's more important to survive than to win quickly.
Once you've taken out around 9-12 wraiths (I lost count personally), Henselt will finally time the stab right and release Sabrina from her agony and break the curse.