Meanwhile, Minerva has sent yes and an only slightly redacted version of the files! And yes, though there is one other candidate I am considering. There are very few power-granters who can customize their abilities to the level where they can directly interact with force-fields, and Magister is one of the two known.
And she'll send the confidential files!
Various useful information:
The slowing wasn't conventional, but everything else we've seen he's already known to use; his conjured hellfire is known to consume good feelings near it, he can instantly rot people near him with a curse, he gives his minions enhanced strength and toughness sometimes. He will absolutely sell immortality to rich people or high government officials in exchange for tremendous amounts of money or his preferred political policies; they are aware of a number of people in various governments (though their exact identities are one of the few things redacted) who have made deals with Magister, and so it's plausible he placed the bounty.
As far as they can tell, he 'builds up' power by causing people who would not be damned to become so, then spends it on his various spells; his precise reserves are unknown but almost certainly enormous given his paranoid nature. Exactly what "damnation" consists of is highly ambiguous (for those people who aren't Catholics), but they think it's based on being a state of where they have done very bad things that they know were wrong and they're glad they did them and don't wish they didn't. Complete list (based off of 12th-century Catholic teaching) here, with confidence levels. They think that his various contracts-for-your-soul are just trickery; the objective is psychological, to convince people that they're doomed whatever they do, but they're really not confident since he puts a lot of energy into signing them.
His known weaknesses are saints' relics and consecrated ground, but he just handles these by avoiding them, which he is really, really good at doing. People have been able to evade him by moving to consecrated ground, and his granted powers have sometimes had the limitation that they don't work on consecrated ground. He's known to be weaker on holy days; his weakest day is All Saints' Day and he then builds up more power throughout the year, with his fullest strength on Halloween.
He is known to have tried to end industrial civilization at least twice by triggering global thermonuclear war; both times the Atlantic Six stopped him. This isn't really classified but, uh, the extent to which some of the people he was manipulating into it got away with no punishment whatsoever is. He can summon very powerful demons, as well as giving granted powers; however, his demons have all his weaknesses but much, much more, and are very difficult to control, so he does it very rarely.
He can teleport in response to attacks that travel at lightspeed (specifically, Minerva's orbital geoengineering laser); it's theorized he has some kind of true precognition power. On which topic, one Halloween he gave someone time loop powers for an evening, so it's possible he can do that to defend himself, though they doubt he can do it on any other day.
His invulnerability does not seem to be that extreme; most of his powers seem to be B-ranked, the problem, as far as they can tell, is that he has a lot of different defensive powers, including extreme fast healing, toughness, illusions, projectiles coincidentally missing him, shields of hellfire that consume attackers, et cetera, et cetera, but this still means that he's essentially impossible to harm. He does not have construct immunity* in any form, so the leading proposed plausible means of killing him is targeting him with a power that "kills humans" or "disintegrates matter" and doesn't care anything more than that. The problem is the possible-precognitive teleportation, which would probably work on that.
They also think there are a lot of restrictions on his spells in general; his contracts often have specific obscure weaknesses or limitations in them, and they might be tools of manipulation, but they also might be genuine limits. Probably, his invulnerability is a large collection of individual spells with individual limits, all of which he maintains out of his spellcasting budget - most likely he needs to keep his soul-trading business going just to maintain himself alive. (Certainly his anti-aging is some form of magic.) "Run him out of power" would be a better idea if he hadn't probably been stably accumulating for about eight hundred years.
He's stupendously arrogant, but he's also very, very good at talking to people; it is probably not worth communicating with him.
There's a "relations with known immortals" section; he is on distant, wary terms with most immortals, they think; Prudence Cartwright has identified herself as unwilling to make an enemy of him, and the Society of Alchemists are known to dislike him but have not declared any open conflict. (A section that has been redacted discusses his relations with another immortal; it's absent, but there's some clues that it was there.)
(*: That's the way that a lot of powers that "work on humans" don't work on robots, summons, or very clever dogs. Type I construct immunity is "not human", Type II is "not organic," and Type III is "plausibly not made of matter.")