All relics possess a power called Scourging the Infernal which damages demons who enter that radius of effect (RoP:I pg 33).
Is the Possession Might Pool or the physical body of a possessed person who enters this radius affected? Does it matter if the Demon within is extending its power over the host or "hiding"?
I think that if the demon is not active, the energumen can enter on the auyra, but any attempt to enter through the possesed being over the Scourging power is impossible if the Might Pool is not greater.
The logic on that, it is than if any demon can simply possess one host, then they would send them upon the Relics and only with one action (one point or none) they could destroy te relic with his powers or something on that way.
Sub Rosa issue 7 has a pretty decent article on all things Possession and this is covered. It would seem that the Energumenus is protected from the Scourging while it remains within the host.
However, that's not to say that it isn't uncomfortable. Gerald of Wales writes of an exorcism in which the possession spirit visibly moved away from the relic as it was brought near to where the demon was hiding within the victim's body. It would seem appropriate to carry that information into the game.