Alright, first, let me say, all the backstory for the Fallen Five and the way you've told it, is genius. I love it. I really think it's awesome. Bravo.
As for the deck itself, pretty solid. I love that you've given the members base abilities that are extensions of their actual cards. I think that's great. The only unsettling one is Siege, whose base power seems like it could get extremely ridiculous to resist, especially in the event that he comes out first. He's a bit OP. The rest of the deck is good too. And the artwork ideas: brilliant.
For the Empty Child, this villain scares me. The idea is awesome and actually terrifying. And I like that we both incorporated "husks" into our decks. But really, I like this one more than the FF. Both are really great, believe me. I think these are awesome. But I'd vote for Empty Child. I like how chaotic it can get with the Quantum Entanglements, but that being said, maybe a little too chaotic and hard to keep track of. The way damage works is interesting though, with the villain damaging itself to do damage at first. And I also feel like when it flips, the villains healing factor is too high.
Siege could get ridiculous, but his slow start tended to give you some time to take him out. But absolutely, if you left him alone too long he got horrific.
In the wrong environment (like the Tomb of Anubis, which has lots of things that damage everyone), Revenant's environmental redirection is brutal.
Every fight we had ended up being a to the wire slugfest where at least 1 (usually 2 or three) heroes went down. Usually trickery was involved in surviving, like using Mist form to let the environment finish the villains off (at that point, the other three heroes were down, and the tomb of anubis was being hateful!)
The Child: I had a lot of fun writing her (it?). In practice, the Entanglements are kind of tricky, but you get used to them. I did a lot of playtesting, and while it helps to have heroes that can eliminate ongoings, you can do the job with tanks, redirection, mist form (or cocooning), etc. I really wanted her to be a villain that rewarded trickery and non-linear combat styles. Or waiting until the right moment for massive attacks!
The backstory for the Fallen Five is brilliant. I love how close each one is to the real hero yet twisted in just the right way. They're so well done in fact, I'd worry that they would be better as something an entire expansion was built around rather than just the fourth villain in an expansion that's already been built by the game designers. They may be too good to win. :)
The Empty Child is creepy, which is great, and between it and the Five, it feels like the Child would probably be a more different play experience than what we've seen before. I think both villains are wonderful, but between the two, I'd vote for the Empty Child.
Your strength is definitely in the story. The Fallen 5 is fantastic, and it's deck seems to complement itself well. I agree with Spiff though, the story is so strong that they deserve an expansion of their own.
The Empty Child is (obviously) a reskin of your Nightmare Child villain on BGG. Strong villain concept. I preferred the original flavour though.
Great story on both, it's easy to see how they fit into the setting. I agree with the other guys, the Child is the best of the two, I really like the Entanglement mechanic - something new and different without being difficult to track.
I enjoyed writing the story for the Five. I tried to find each hero's defining moment, then have it all go wrong. Or draw the wrong lesson from it.
I may have gone a bit overboard in the backstory department, but I can't write a villain (or hero for that matter) until I know a bit about them and who they are. For some, once I had the name everything slotted into place- like Seige- and for others, it was just an extension of their existing story, but taken farther and farther over the line. Truth be told, I even have a hard time PLAYING heroes unless I know something about their personality (I used to get really frustrated playing Bunker until I learned more about his tactical, analytical personality).
Actually, the most fun I had was in trying to make their individual cards as close as possible to the heroic versions. And naming them as close as possible to the originals. Calculated construct is my favorite!
As for the Child, I needed to re-skin the original, since the original was dream based. The original version also had some problems that I got to work on with a fresh eye, since I was coming at it from a different angle and perspective with the time focus. I also got to have a lot of fun being excessively creepy with the card art/text. People really wanted to win against it, just based off of the flavor, which sometimes led to them wanting to get it over quickly and acting impulsively...which made it all the more fun when the cards punished them for it! And all the more awesome when they finally got to make those last big punches to finish it off!