Thanks all.
Arthur: Yes, it was a single botch.
More about the dead bad guy mage here: https://forum.atlas-games.com/t/raiding-an-enemy-sanctum-traps-obstacles-requested/10263/1
I shall henceforth call him "the Rogue Mage."
The Rogue Mage was insane, with at least two specific personalities: His younger, innocent self, riddled with guilt (Maris), and his self-righteous, over-confident, angry self (Talos). There are other personalities in there too, different personas that he assumed as he traveled the Mediterranean throughout the years, assuming different roles, trading with covenants under fake names, etc. He was Driven to keep an ancient magical artifact a secret. When the players accidentally stumbled upon it, they became his focus. The players were able to take him out when he got distracted by his own anger.
I don't want the ghost to be a spirit that can have new thoughts, etc. It would be a shade: Acting from the point of view of the Rogue Mage's final moments (which is to destroy the PCs before they are able to disturb the ancient relic.)
I like the idea of turning this into a longer story arc. The lich is interesting. The infernal spirit is also interesting to me. In life, the Rogue Mage used a lot of mind-control magic to get his way. This is certainly a dark path, and would attract the legions of hell. These could all be combined into the Lab Rat being enslaved by the the Rogue Mage's ghost, to work on a big project to transfer the spirit into the dead body, or even an infernal resurrection of sorts, where the soul (not just the ghost) would go back into the body.
Has there been anything published in the RAW about lich magic?
How does the Aegis affect this situation? If the talisman is an arcane connection to the ghost, is the ghost able to get in to the covenant, but it's powers are dampened by the aegis?
Finally, if I do go with this longer story arc, I'd love to hint at it with some silliness at the beginning of next session. Do you think it makes sense that the Lab Rat's mind would be transported into the dead body?
Thanks again!