Annnd here's the choice, people. A spirit (evil or otherwise) has been trapped down here by magic and wants to be released to save the children of the swamp village from the ladies (crones) of the woods. Helping the spirit will trigger the optional sub-quest below. If you start it, the spirit will create a path out of the hillock for you to retrieve the items needed for it's ritual. If you decide the spirit must die, you skip the sub-quest and go straight on to a relatively easy fight against the spirit. Skip to the sub-quest below if you decide to take that path, otherwise prepare to take on the spirit.
Once the battle begins, the spirit will shield it's heart in a dome of energy. When it does, three endrega workers will emerge from the nearby walls and attack you. They do relatively little damage and are quite suceptible to the igni sign and fast attacks from your silver sword (they are weak enough that you don't really need to prep for these guys). Use quen if by some twist of fate your life gets low, but otherwise simply hammer away at them. Once all the workers are dead, the shield around the spirit's heart will vanish. This is your opportunity to get some damage in. Hack away with light attacks at the roots protecting the heart, then slash away at the heart itself.
After taking around half damage, the process will repeat again. The heart will shield itself and three more workers will appear. Resume the worker slaughter, slash the roots and the heart again when they become unshielded and you will emerge victorious. If you took this path, skip the sub-quest below and move onto the next objective after it.