reading The Path to Heaven.
Emp Children feature quite prominently.

So, did Fulgrim UNpossess himself just by looking at 'The Portrait'? But isn't that how he got possessed in the first palce, Dorian Grey-style looking upon the summation of his sins embodied in the portrait? This sounds like it is eluded to in Path to Heaven. Is Fulgrim still Fulgrim? Or is he actually possessed by whatever Slaanesh sex-monkey that's riding him like a pony?

The books seem to contradict, and, according to the 3rd Legion, they have no idea where he is or what's happening to him.
In 'War Without End' - there is a Fulgrim short story, which explains his whereabouts, but not if he is possessed or not.

Could anyone shed some of the Emp's Light on this, please.