I had a large thing written up about this explaining it but then I hit the backspace, it didn't post and now I"m annoyed so I'm not writing it up again. Basically, here's what is important:
I made this for my group just to have fun with since we're all fans of the Shin Megami Tensai series and it also has Nyalarthotep and Nodens in it--so I figured why not?
The Major Arcana are referencing the Shadow and Persona by having the split choices--one choice gives a boon to all Investigators while the other gives either one to the First Player or screws over everyone BUT the First Player. This is also a play on the notion of drawing 'Upright' or 'Reversed' cards in Tarot readings.
Blessings of Nodens is included in this Guardian because in the game Nodens was (spoilers) an apsect of Philemon.
The Major Arcana deck can be used without using Philemon.
The Tarot Deck is a Unique Item.
Finally, this is ideally played with Nyalarthotep as the Ancient One. Even then, this Guardian will make things easy unless you include other elements to increase the difficulty. I recommend using the Black Pharaoh to even the odds. This Guardian was designed to give the players a more relaxed but more competitive and 'back-biting' version of gameplay.
There are some wording issues in these that I've picked out as I was re-reading them and I'll be putting up the corrected texts once I get a feel for how folks like or dislike the idea of this set.