So the Daemon rule confers the following special rules: Fearless, Fear, and a 5+ Invulnerable Save.
The Grey Knight FAQ asks which units are considered Daemons. I think it's important to note that Daemon used in the FAQ question is in bold. The answer is all units in the Daemons codex, except spawn, and Daemon Princes, Possessed, Greater and lesser daemons, and any model with Daemonic Possession.
The Defiler comes with Daemonic Possession so it is considered a Daemon. Does that mean it should have had a 5+ Invulnerable save this whole time?!
The Grey Knight FAQ asks which units are considered Daemons. I think it's important to note that Daemon used in the FAQ question is in bold. The answer is all units in the Daemons codex, except spawn, and Daemon Princes, Possessed, Greater and lesser daemons, and any model with Daemonic Possession.
The Defiler comes with Daemonic Possession so it is considered a Daemon. Does that mean it should have had a 5+ Invulnerable save this whole time?!