To complete the "Defeat an Essence Monster that is Possessed" PoE Mirage challenge, use an Essence Scarab along with the Torment Scarab of Possession on your map device. Alternatively, use the Seance Atlas passive node to guarantee possessed monsters. Find an Essence mob, lure a Tormented Spirit into it, or wait for the possessed effect, then kill it.

Key Strategies for Completion

• Best Scarab Combo: Use an Essence Scarab (Area contains 3 additional Imprisoned Monsters) and a Torment Scarab of Possession (Rare Monsters in Area have a quarter chance to be Possessed by up to 3 Tormented Spirits).
• Atlas Passive Tree: Allocate the "Seance" node, which grants up to five rare monsters in a map to be possessed.
• Alternative Method: Use the "Essence Scarab of Calcification" to convert all rare monsters to imprisoned, then hope they get possessed, or use maps with "Tormented Spirits" mods.

Tip: Ensure you do not take the "Speaker of the Dead" node, as it can hinder the possession process.