Was wondering if any canny underdark DM's could enlighten me on what would happen if someone was overly exposed to this particular type of radiation. And any other general information about faerzress is good.

All sorts of things. Maybe nothing.

The radiation explained many Underdark phenomena like the innate magical abilities of the drow, but was largely pushed to the side in 3e. It's a handy tool for storytelling, but it's probably fair to say its effects are somewhat unknown and unpredictable.

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What Iron said, pretty much. If you're interested in learning more about that, though it may or may not be canon as far as Amian lore is concerned, you can read the Underdark supplement for the FR campaign setting.

The most commonly-known effects of the Faerzress are: any creature in an area of faerzress receives a +4 bonus to their will save against divinations (including Scrying and Greater Scrying) and spells of the Teleportation subschool of Conjuration don't work reliably for distances over 1 mile.

I'm fairly certain that Amia does not use or acknowledge Faerzress for its setting, so I wouldn't worry about it for here.

For elsewhere, the material already cited should be your best bet!

There probably aren't any notable concentrations in the popular and well-visited areas of the Amian Underdark (because it has never featured prominently in any DM story that I know of), but I've never heard of it being retconned either. It's a pretty integral part of the FR Underdark, after all. We have retconned very little of canon lore, which is an achievement to be proud of considering all the new lore Amia has integrated into the setting over the years. We've even kept both cosmologies relevant. It's probably wisest indeed to keep it as a somewhat obscure option to whip out when necessary.

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Attitude may have changed about it since, but I recall some suggestions I made about Faerzress a while back being shut down because Amia doesn't make use of the mechanic.

Anyway, here's some other threads related to it, maybe interesting/helpful thought-food in them!

Faerzress Lore
Drow and Spell Resistance

I think most people are well aware of my opinion on the Magic Radiation Of Underdark Power-Granting by now....

Here's your answer.

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Davion Telemos - Monk of the Four Winds
Korthan Isharnos - Dragon Shaman of Thunder Spirit Zamasham