I wondered when a non-creature, build-around cards were going to make their way into Avacyn Restored:
Okay, so it may not necessarily be a build-around because it may also be useful as a general control/board sweeper type card for a Mono Black Control deck. Descent into Madness immediately evokes memories of cards like Smokestack and Braids, Cabal Minion. All-in-all I think this card is very good and I'll explain why...
All you need in a deck that wants to abuse this card is to stay ahead in the permanents and/or cards in hand count. You need to make sure your capacity to exile permanents and cards in hand is greater than your opponent by the time Descent into Madness enters the battlefield. The best way to accomplish this is to kill as many opposing creatures and force your opponents to discard as much as possible. And luckily for you, black is very good at both of these things. The other reason why I think Descent into Madness is very good is because you get to choose when to turn it off if the advantage ever falls out of your favor. The enchantment itself is a permanent which can be exiled to itself so it has a built in mechanic to turn off its effect.
The only thing left in judging this card is to compare its effect to its cost. At 4cc I think it would definitely see play and would "obviously" be a good constructed card. But as it stands now at 5cc, it's a little harder to judge and is kind of in that "maybe territory" which is completely dependent on the current metagame. If the meta is slow, Descent into Madness seems good. And if the meta is fast, it may hit the battlefield 1 or 2 turns too late.