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When Khorne Daemonkin first came out, the fact that Daemonic Instability was replaced with Fearless meant that for the first time, you could have non-daemonic Characters join Daemonic units, so long as those units came from Khorne Daemonkin. While this leads to stupid rules interactions (For example: A Herald of Khorne from Codex: Daemons cannot join a unit of Khorne Hounds from Codex: Khorne Daemonkin due to the former having Daemonic Instability), it also opened up combinations that had not been previously seen. When the Black Legion reprint came out with new formations and added the Cyclopea Cabal, the "Cabalstar" become a new build for Chaos.

One thing I've wondered is: What if Daemonic Instability's "Cannot join (or be joined by) models without this rule" segment was removed altogether, thus allowing for proper Battle-Brother interaction between Chaos Daemons and their allies? What new power combos would become available?

One "stupid" combo I could think of would be joining Exalted Flamers of Tzeentch to small units of Plague Zombies. The Zombies would grant a Exalted Flamer Slow & Purposeful, allowing it to fire-on-the-move, while the Exalted Flamer becomes a proper gun.

Are there any others I'm missing?

A herald of Nurgle would become an easy source of both shrouding and FnP for any unit in the game.

Likewise a Herald of Khorne would become a source of Hatred and Rage.

There's a couple of other nice little combos but not really worth mentioning.