I looked into this for ya, and found this:
The cards conceal many secrets. I cannot claim to have learned more than a fraction of them myself; this pamphlet contains a few of my discoveries.
The Dereth Tarot consists of two kinds of cards: Lesser Trumps and Greater Trumps. The Lesser Trumps are similar to the familiar playing cards used in various games, consisting of 14 ranks in 4 suits, namely the Ace through King of Swords, Skulls, Scepters and Drums. The Greater Trumps are not in any suit, and each has its own title and description.It is said that in the past there were many more trumps extant, but only twelve are currently known.
Greater Trumps may be combined together magically to form melds. Only a few combinations of Greater Trumps in the many millions of possibilities can be combined in this manner, and short of trying them all, there is no way of knowing how many valid combinations exist. Many Seekers keep their discoveries secret, but I will break the seal of silence to give a simple example: the trumps called 'The Living Moon' and the 'Shattered Moon' may be combined to form a simple meld known as 'Twin Moons.' It is said that the more trumps which are combined to form a meld, the greater the power.
Once a meld is formed, it may not be broken apart. The Greater Trumps involved in the meld are lost forever for other purposes. In fact, melds are so magically charged that they may not even be traded from one Seeker to another; a meld is an expression of personal spiritual power that will forever be attuned to the maker.Melds may be used to transform ordinary Lesser Trumps into powerful tokens called Fate Cards. Some Fate Cards can convey their power to items such as arms and armor, and other Fate Cards provide a magical benefit to any who care to meditate on their mysteries.
The interaction between melds and Lesser Trumps is a mystery in and of itself. The Fate Card which results from the application of a meld to a Lesser Trump is to some extent determined by the meld, to some extent determined by the Lesser Trump, and to some extent, apparently, by the individual psyche of the Seeker performing the magical operation. While I cannot claim to have a complete understanding of this mystery, I will say this: Many Fate Cards are determined in some manner by the suit of the Lesser Trump in combination with the meld. Fate Card power seems to vary widely from one operation to another. It is my belief that the rank of the Lesser Trump plays an important part in determining the power of the resultant Fate Card.
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It sounds like there are different recipes that you can "meld" together, to create "fate cards" that will instill buffs for yourself/weapons/armor when used, and I've also seen people report significant XP rewards too. I'm not certain if the effects are permanent or temporary though
You meld them together using these craft recipes:

failed attempts do not consume the cards.
I'm guessing the greater number of cards used, the more powerful the buff (and consequently the more difficult the combonation).
I'll keep an eye out for recipes, and post em up if i find some.
-Chee