To complete the "Defeat a Unique Map Boss that is Possessed while in a Ritual" PoE 3.28 challenge (part of the Cross Contamination set), you must ensure the boss is both possessed by a Tormented Spirit and within an active Ritual encounter at the moment of death.

Preparation

• Use a Tormented Scarab (Area is haunted by 4 additional Tormented Spirits) to ensure ghosts are present,
• or Choose map with mods: Unique Bosses are Possessed

List of Ritual Scarabs

The Process

1. Locate a Ritual altar near the boss room.
2. Lure the boss into the Ritual circle and kill it.
3. Activate the Ritual, which will cause the boss to respawn within it.
4. Ensure the boss becomes possessed by a Tormented Spirit (often happens naturally if the boss is possessed or if using Tormented mechanics).
5. Kill the possessed boss while the Ritual is active.