Headrotten and Headless Husks:

Headrotten are the primary monsters inside the Headhunt zones. You'll need Diablo 4 Items to kill these monsters to gather Grim Favor, which can be exchanged for seasonal rewards such as Witchcraft Powers.
Headless Husks are Rare and Elite variants of Headrotten that drop Fugitive Heads and Occultist Gems. These heads are important for progressing in the Headhunt content.

Grim Favor and Raven of the Tree:

After collecting Grim Favor from slain monsters, you can turn them into the Raven of the Tree. The Raven acts as a go-between for the Tree of Whispers, where you can exchange your Grim Favor for Diablo 4 Items for sale rewards.
The Raven is generally easy to find, located near the center of the Headhunt zone, and marked by a Raven icon on your map. Simply interact with the Raven to progress your rewards.