Demons Deck

Minimum Raw mats for Shadowy Tarot x5 that make the Demons Deck are:

10 Violet Pigment
10 Ruby Pigment
5 Heavy Parchment

Since milling rates vary based on the iLvL of the herbs milled, the number of herbs you will need will vary greatly. The lowest herb in this pigment bracket is Firebloom (2.44 violet pigments per mill, and 0.32 ruby pigments per mill). The highest herb in this pigment bracket is Blindweed (2.83 violet, 0.56 ruby per mill.) That gives us the following statistical averages for raw mats per deck:

20.49 Firebloom = 10 Violet Pigment
156.25 Firebloom = 10 Ruby Pigment

17.49 Blindweed = 10 Violet Pigment
89.29 Blindweed = 10 Ruby Pigment

If milling other herbs in this pigment bracket it will on average require somewhere between these top and bottom numbers in mats. In other words since the ruby pigment is the limiting factor you will need 4 and a half stacks of Blindweed per deck of 5 cards (and will have a bunch of violet pigment left over).
Luck is playing a big role here in both milling and getting 1 of each card. This is the bare minimum so you will need more mats than this if just making 1 deck. This info is more for someone planning to make large numbers of decks (perhaps for Insane in the Membrane) and want to cost out the raw mats in bulk.

This repeatable quest gives 25 Darkmoon Faire rep and the lvl 40 rewards are now BoE (used to be BoP). The deck can be used at ANY TIME. The Faire does NOT need to be in town to use these decks.

Note that to be able to choose one of the BoE rewards, the item has to be intended for your class. I just did this on a hunter, figuring I'd be able to pick the 2h axe or dagger, to learn the transmog. I wasn't offered anything at all though, since none of the weapon types are considered native to hunter. You can use the Wowhead function above to see what classes will be offered what weapon.

You cannot as all of these weapons require level 40.