Greater Daemons and Possession
An alternative means of remaining in the real universe requires that the Daemon takes possession of a living body.
The preparation required to summon a Greater Daemon is considerable, so if it is to be effective the ritual must be performed before battle. A single Aspiring Champion model in the army will be acting as the vessel for the Daemon (known as the Daemonvessel or sometimes Daemonhost). The exact model should be noted down before the game, there is no requirement to identify it specifically to the opponent. A Greater Daemon may only possess a model bearing the Mark of Chaos Undivided or the Mark of its own patron god. A Bloodthirster could only possess a model with the Mark of Chaos Undivided or the Mark of Khorne for example.
Until the Daemon manifests, the host will fight with the Strength characteristic of the Greater Daemon itself (note that whatever wargear the model may have, its effective Strength can never go above 10).
At the start of each turn after the first that the Daemon's Host is on the battlefield, the owning player chooses either to resist the possession or invite it. Roll a D6; if the choice was to resist then the Daemon only takes possession of the vessel on a roll of 6. If the choice was to invite possession then the Daemon possesses the vessel on a roll of 4 or more. When this occurs replace the vessel model with the Greater Daemon model. If Possession occurs while the vessel is inside a transport vehicle or bunker it is assumed the host staggers out screaming before the nightmarish transformation occurs. The Aspiring Champion replaced is treated as a casualty.
If the enemy kills the host before possession occurs, the Daemon will automatically take possession of the vessel's body at the end of the player turn in which its host is killed. Place the Daemon model on the position occupied by the late vessel, in contact with the same enemy models (if any).
The possessed flesh of a dead Chaos Space Marine will not sustain a Greater Daemon forever, so at the end of each Chaos turn that the Greater Daemon has started on the battlefield after possessing a dead host, roll a special Instability test for the Daemon using 3D6 added together. If the sum of the three dice is greater than the Daemon's unmodified Leadership it suffers the difference in wounds with no save possible. A Daemon that is lost this way is considered to be dead for victory point purposes.
Errata — Chapter Approved: Codex: Chaos Space Marines Q&A (v3.0): A Greater Daemon does not have to take Instability tests if its Chaos Space Marine host was still alive at the moment of possession — Instability is only a risk once the Daemon has possessed a host that was removed as a casualty. Any circumstance that causes the host to be removed as a casualty brings the Greater Daemon into play, representing the Daemon harnessing even the forces that destroyed its host to manifest and take revenge before being banished. The one exception is if the Daemonhost never gets onto the table or is lost from it entirely (eg it falls back off the table, or is lost in the Warp while Deep Striking) — in that situation the host effectively becomes a casualty along with the Greater Daemon, and neither enters play.