The magi were gathering to cast the Aegis of the Hearth, when one of the grogs, having second sight, shouted out about a shadowy figure looming over one of the elderly magi. The magus himself, Oswald of Elk's Run in Hibernia, follower of Bonisagus, was too sickly to take much notice, but other magi were concerned. None of the magi were able to see this figure, not even with a serious InVi spell. A little later she saw a second figure, which also one of the magi, with second sight himself, could see. He seemed to be identical to a person who, posing as a follower of Bonisagus, had introduced two magi to a demonic cult, and was invited into the Aegis with a token two years prior by a third magus. The invitation was revoked when the three magi discussed the matter, but maybe he had stayed inside the Aegis since then. It seems that the two immaterial beings took part in the Aegis ritual so as not to be hampered by it for the next year. While there is no proof that they are demons, there are some clues pointing that direction.
So far the uninvited residents have not done much harm. One of the magi got an urge of avarice and stole two books from the private collection of a senior magus, and another was approached by one with an offer of help and support to assume leadership and «make the covenant great».
Now, the question. You know that there are two unwelcome, alien entities living inside your covenant, taking part in the Aegis ritual, but only one or two people can see them, and nobody has any spells to affect them. What do you do? What can you do?
Yes, I realise that there may be questions over the interpretation of the infernal and the aegis, and I am happy to discuss those at well.