Needing help, I can't find this in any book

[Anderlith]

So a year or two ago I was skimming through some D&D books & found a really cool hazard type effect. It was rules for areas were magic has gone weird & can hurt casters if they cast spells. I think it was kind of like a magical fallout zone. When I saw it I made a mental note to remember this when I went to run a new game, problem is... I don't remember where I found this. :( Can the playground help me out?

[Kelb_Panthera]

You're probably looking for wild magic zones. There's definitely a description in FRCS and I think there's one in complete arcane.

If you don't have FRCS and/or it's not in complete arcane, the effect is just a localized version of the wild magic planar trait, described on DMG pg149.