I like the idea of Fallout as a mechanic, but as @FanOfMostEverything said it should go in a set that has a LOT of graveyard hate.
 
Something like:  
RadAway (2)(G)  
–Sorcery  
Target Player exiles the top 5 cards of their graveyard. Draw a Card.
 
or  
Rad-X (1)(G)  
–Instant  
Until end of turn whenever a card is put into it’s owners graveyard exile it instead.

@Damaged @FanOfMostEverything
 
Forgot to type “from the graveyard.”

Fallout should be a lingering effect, so I’d go with either @Bardic_Knowledge or @Damaged
 
Also, Power Armor equipment that adds x/x (depending on how much you want the mana cost to be for balance) and immunity to Fallout effects.

@The Hittite  
Where can this ability be used from? While casting? While in play? While in graveyard?

@The Hittite  
I like this version. It doesn’t forcibly tie the effect to a global -1/-1, which allows for greater flexibility. Plus it turns Geistblast into a radioactive laser made of ghosts, which is amazing.

Fallout X — When [this card] is cast, exile it with X time counters. While [this card] is exiled with time counters, all creatures get -1/-1. At the beginning of your upkeep remove a time counter from [this card], if no time counters remain, place it in your graveyard.
 
That way “fallout” gives a lingering effect that eventually fades, can be fed counters to persist longer, or can be countered by removing time counters from it.

Idea that just popped into my head:
 
Fallout – When you cast this card from your hand or graveyard, you may pay (mana). If you do, exile it and put a (minus one/minus one) counter on each creature.

Okay, brainstorming here.
 
What about  
Fallout – 1B, exile Balefire Bomb: All creatures get -1/-1 until end of turn.

The fallout effect is too good. Although is would make an easy way to counter infect.

@The Hittite  
Maybe if Fallout effects mean that the card is removed from the game instead if sent to the graveyard without having paid it’s mana cost to play it. Crippling Addiction is also ridiculously strong, as you can pay 1 mana every time a player starts their turn to make them burn off all their non-land cards.

@FanOfMostEverything  
That’s fair, I hadn’t really thought of some of that.

@The Hittite  
The key problem with fallout is that it doesn’t care how the card gets into the graveyard. Self-discard effects become incredibly powerful. Milling becomes either a terrifying prospect or the route to victory. The lack of any limiters or qualifiers means that blue and red decks will likely run these with looters to gain access to off-color effects. Yixlid Jailer becomes a must-have.
 
Granted, these are the rares, so they may give a skewed portrayal of the mechanic. Still, they paint a stark picture. Also, note that cards don’t actually have ability words—they’re italicized for a reason—so Necromantic Resurrection’s second ability does nothing.

@Bardic_Knowledge  
I considered making it an enchantment but decided on sorcery because  

@Bardic_Knowledge  
Fallout’s an ability word, so it probably just means each fallout card has some impact on the game while it’s in the graveyard. The Judgement Incarnations could be said to have fallout abilities.
 
But yeah, unless this is in a set with a lot of graveyard removal effects, it’s disgusting, especially in multiples. Flavorful, yes, but terribly anti-fun.
 
@Amateur-draw  
A balefire bomb is a megaspell, but not all megaspells are balefire bombs.

What”s the difference between a balefire bomb and a megaspell?

@Sparkle_Dusk  
Yeah, I’d probably have this as an enchantment that blew up the field on entrance and just have the -1/-1 as a static effect.
 
That being said, everything with “Fallout” would give all creatures -1/-1, including your own. Ergo, if you have four of this in your graveyard all creatures get -4/-4. So you would either have to build your deck around a lack of creatures, or build it around +1/+1 static effects to counteract the Fallout.

oh, fallout is so op!!!

Ah, best used with ways to pitch cards into your own graveyard. Blue-black control would be my deck of choice for such an animal.

This…leaves me with a dilemma.
 
Do I run this over Night of Souls’ Betrayal in Jund…