The available trinkets
These are the current round of Darkmoon trinkets you can get in Cataclysm:
Darkmoon Card: Earthquake This trinket is obviously intended for tanks, but it's actually kind of mediocre. While dodge is certainly an attractive stat for tanks, the on-use ability is fairly weak. Most tanks will reach for more stable stat boosts and long-term mitigation.
Darkmoon Card: Volcano How much does your class care about mastery? If you care about mastery and you can pick up Volcano inexpensively, then it's great. But you should be aware there are many other trinkets with greater or equal mastery. The proc is interesting and attractive for many folks, though, so we might see Volcano around for a while.
Darkmoon Card: Tsunami The Tsunami trinket is obviously intended for healers, especially those who are interested in increasing their combat regeneration. The sad news is that there are better options for most classes, but you can't pick up those options from the auction house. Tsunami is good for folks getting started, but you might not get the whole expansion use out of it.
Darkmoon Card: Hurricane This comes in both a strength and an agility flavor. In both these cases, the card is a good head-start trinket, but it doesn't seem to have the lifespan of previous Darkmoon decks. The proc provides a burst of damage, and the cards have an overall stat bump ... but the trinket options in Cataclysm are pretty expansive compared to what we used to have.
Overall, the Darkmoon trinkets are "pretty good" and "very helpful" but not necessarily the best item for virtually every class the way they used to be. And that's a good thing, since it's a little sad to not upgrade a piece of gear for two years. Darkmoon cards are worth the money if you have money to burn, but not worth paupering yourself over paying insane auction house prices.
How you make them
The thing about Darkmoon cards is that they're incredibly expensive to make. Check out Basil's explanation of beating the RNG in making decks
. Each deck has a "suit," and you need all eight cards of that "suit" to combine into a deck. By Basil's experimentation, a scribe has to create 52 random cards to get enough for a single deck.
That's a lot of work. This is what leads to Darkmoon decks being so crazy-expensive; someone has to grind through a lot of herbs, a lot of scribe time, and a lot of RNG to end up with a finished product.
If you decide you want one of the decks, your can either buy a complete deck on the auction house, roll a scribe and get to work, or try to find the individual cards "on the cheap" on the auction house. Whichever method you choose, good luck.
The final step
The final step when you've collected all eight Darkmoon cards is to go visit the Faire. The assembled deck provides you a quest to help you find the turn-in. The Darkmoon Faire travels and is only available for a limited time, so it's best to work on assembling a deck during the downtime between Faire appearances. (The interest is lower, then, and thus decks tend to be cheaper.)
And that's what you need to know about Darkmoon Faire decks.
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