Greater Daemons can appear on the battlefield by taking over the body of one of their followers. Starting with the second turn, instead of rolling the dice to see if the Greater Daemon appears on the table as described above, check to see if the Daemon has possessed a model and enters play. This check is made at the start of the assault phase, either before or after summoning any other type of Daemon.
To see if a model has been possessed, pick any character model in the army apart from another Daemon and roll a D6. If you roll a 6 the model has been possessed as described below. Note that no modifiers apply to this dice roll. If you fail to roll a 6, pick another character model, and roll the dice again. Keep on doing this until you roll a 6, or have rolled for all of the character models in the army. Note that all characters must be rolled for, but the Chaos player gets to pick the order the dice rolls are made in. If no character is possessed, the Greater Daemon does not arrive this turn, but you must check again, following the same procedure, in the next turn.
If, or once, a model is possessed, simply remove the Chaos Space Marine model and replace it with the Greater Daemon model. The model that has been removed is destroyed in the process and counts as having been killed for victory point purposes, etc.