There's a lot I like about this supplement, and one thing I passionately dislike. I'm struggling to give it a 4/5, instead of a 3/5 - however, since my dislike is a thematic gripe I'll leave it in this review and let the reader decide.
Imperial Mysteries goes CRAZY WAY OVER THE TOP with Magic. It completes the Gnosis chain and Practices to a degree not even hinted at in the core book. I was always a bit disappointed that the core mage didn't really address what went on with Arcana once you got above Gnosis 5... it seemed such a low number to cap development at. This blows the lid off it, creating a system which does not even pretend to balance what Gnosis 9 vs Blood Potency 9 means. It's what the final supplements of Changeling hinted at - the path to the final transformation and ascendancy for the story.
It does a lot of things amazingly well. You don't have to play with any of the mechanics in the book to get a wealth of ideas about what Archmasters might want, involve themselves in, or leave behind as mysteries and echoes. Using the systems you can give players sufficient structure to play at the highest levels, while retaining a sense of cost and consequences.
For those of you who are familiar with Unknown Armies, it truly takes it to the Cosmic Tier - where you actions /literally/ reshape the world.
....This is both a good and a bad thing.
For me, it strays too far from what made Make: Awakening really amazing - revealing the secrets behind the curtain long hinted at... to be some sort of crossbred UA / Mage: Ascension bastard love child. They even CALL this highest tier "The Ascension War." For me, it really feels like the developers stepped broadly away from one of my favorite things about Awakening. Here's what I mean:
"Once understood, Imperium is a revelation that forever
changes a Seeker’s outlook on the universe. Before archmastery
she might have assumed that ultimate power lay embedded in
an objective secret history, that Atlantis’ ruins hid beneath the
waves in a defi nite location, aged grimoires always contained
the same spells, and the Exarchs seized power in the distant
past — not a present erased by their triumph. Everything might
be true, if the Invisible Masters permit it."
While the text then goes at length to say a bunch of the usual stuff around "well, it's your game, so maybe that's not how it REALLY is, just how it appears to the archmasters as they fall deeper into their Mystery play..." - functionally, the entire core assumption of this supplement is that there is no ACTUAL TRUTH to the Supernal and that the "Fallen" or "Phenomenal" World which everybody is a part of is just the result of the constant revisions and edits of the Archmasters. It's the Consensus of M:tAscension, just only certain people count now.
Why this bothers me is that what I did not like about Ascension was that the postmodernist setting essentially resolved all mysteries and secrets to "Whatever you want to believe, man." Indeed, as a Mage progressed their Paradigm... the thing which made them unique and interesting... became LESS important. Whereas in the more Gnostic setting of Awakening, there was some truth the universe - as filtered and shattered as it might be - so the players might truly have the sense of approaching Mysteries and making progress toward something ever higher. Well, now "higher" just reveals that nothing they discovered was actually True and the only thing that matters is your ability to exercise power.
Indeed, Archmages spend most of their time seeking "Quintessence" - a mystical quality of experiencing something... which symbolizes how you /feel/ about the universe and the spell you want to cast next.
In the end, instead of pointing the way to even higher truths, greater mysteries and the shaping of your soul into an ever more refined vision of your channel between the perfected world and the phenomenal world... the curtain is pulled back to reveal that the universe is comprised and shaped of nothing but the personal politics and ideological agendas of a few score people.
So, I like the book. It's got a lot of meat I can use. However, for anybody who was as excited by the Gnostic Mystery of the game as it's been portrayed so far, I feel you're bound to be a bit disappointed by the direction they took it.
Finally, and as a somewhat petty thing, the art direction of the book was a bit disappointing to me. A /lot/ of art is recycled from previous books, and after so many truly beautiful and evocative covers I feel that the sketch pencil / watercolor which we have here does not fit well.