Erebus made a possessed Pariah during the HH for example, which normally shouldn't be possible.
Shouldn't the pariah cause the possessing daemon to just... cease to be?
The pariah's presence should banish it back into the Warp, which is what normally happens to daemons in true null zones, but Erebus managed to bind a daemon in despite that, at which point the combination became something else, something worse. Spear, as I recall it being called, was in addition to being a potent assassin and a daemonhost, a psychic mirror. He could repel psychic energy back at his enemies.
It is worth bearing in mind that Erebus may well be the greatest daemonologist ever to exist, possibly even greater than Lorgar. A feat like this required the blessings of the gods, immeasurable will, hatred and ambition. I dare say no one else could do it, and even for Erebus it was meant to be a great challenge.
I've not yet read the HH novel the whole daemon-pariah thing happens in, but I've heard mixed things about it, so I'm not sure if it's any good.
EDIT: It's always good to remember that many 40k characters are defined by being abnormal or superior to others. Like Typhus, the only marine to fully bond with the Destroyer Virus, or Kheradruakh the four armed mandrake character with an unrivalled connection to the shadow dimension. Or the Blood Angels character who overcame the Red Thirst and the Black Rage.
A possessed Culexus Assassin isn't really that far out there for characters overcoming their inbuilt failings and weaknesses or breaking the bounds of what is normal for the setting.