General review on the challenge decks.
What are they?A block event, the dealer used a special event deck against you (or you just played by your self and reported winning), these decks had no londs and used specialized cards to dictate the deck's behavior. If you manged to beat it you picked up a special event card (woot). Following the novella you could progress through the story in your own card based way.
Deck #1, the HydraThe hardest deck out of them all. Easy/Normal/Hard modes based on the number of starting heads. The default head is a 0/3 creature you can attack directly as if it were a player. Killing a head awarded a Hero's Reward (typically +2 life per player) and the Hydra draws two cards and if any number of them are another head card it goes into the battlefield immediately.
The Hydra draws one card per turn which is typically another normal or elite head, turn delay (heads are indestructible or you cannot cast spells, lasts one turn), or deal even more damage. Some of these cards are pretty cool, like Torn Between the Heads deals 5 damage but taps two heads so its like those two heads really are too busy trying to tear you apart. At the end of it's turn, it direct deals 1 damage per head (2 per for elites) no matter the player count. You win if at any time the Hydra has no heads in play at the end of the turn (ie wrath of god may not kill it since new heads can be drawn and put on the field).
Honestly it's my second favorite of the three. But all three heavily favor creature based decks which kind of robs the fun. And since none of the creatures actually attack many anti-creature methods such as Propaganda and Fog have no effect.
Deck #2, the HordeEasy/Hard comes in as the number of turns you take before the Horde deck starts taking it's own turns. It gets 2 cards a turn which typically are haste & always attacking creatures. It has artifacts that allow it to draw more cards, "spells" to give the horde haste and requiring additional blocking, and a nice global wipe that hits all creatures for 3 damage (kill everything the horde has out).
You can't attack the creatures directly but for each point of damage dealt to it's "player" it mills a card. The deck loses when it's milled out. This one does have an official multiplayer rules, it draws 2+(1 per player) and you share life. This is where I got the multiplier adaption for the Hydra from, 1 card per player and shared life total. The deck is pretty easy thanks to having up to three turns before it. I like it because the rest of the casual players I know are built to handle it (ie creature based).
Deck #3, kill the stupid SatyrXenagos's deck is almost a boring capstone to the Challenge Decks. Hes a 6/5 Indestructible Creature and you have to kill every single one of his creatures before you're allowed to remove him form the battlefield. Spellside of his deck is more varied, it's the all-arounder really. Sometimes Xena attacks, sometimes his creatures do, nothing really fancy.
Since his creatures don't attack every turn you'll have dead turns where it sits there doing nothing. Maybe it summons a 1/3 creature meh. Easiest of the three and it just lacks a unique feel to it.
RewardsYou get a special Hero Card you can use against the Challenge Decks to make things a little easier. They sit in your Command Zone and you begin play with them. They range in usefulness, like the Hydra's reward is The Slayer (start with +7 life) while Xena gave a Equipment card I think. They are not legal for normal play.
I ordered all the ones I missed and I plan on copying the Vanguard Cards (way to expensive to buy in real) and coming up with my own over the top Vanguard/ELH/Planechase/Hero format with all but the ELH element randomized.
OverallI'd suggest picking them up, they are only like $10 bucks a deck when I ordered them. Co-Op play is always fun with no butthurts afterwards for losing so overall it's nice to break them out every now then, specially after rebuilding a deck and you want to see how it'll fair against a creature deck (zerg rushing horde ftw!)
Read more about the Hero's Path Event you missed
here. Then just order the cards and run your own