It's all an aboleth plot!
The rift reached the depts of the underdark, opening wide to an underground sea where a branch of the Abolethic Sovereignty was busy manipulating a pocket of Spellplague and covertly experimenting on creatures in and near Neverwinter.
-- "Abolethic Sovereignty", Neverwinter Campaign Setting, p.90
Predating the MMO by more than a year, if you can get your hands on a copy of the campaign setting book, it'll have at least some details that tie into it. Though don't expect to see everything; further development of the MMO has tied in more to the release of various 5E products like the Hoard of the Dragon Queen or the Dungeon of the Mad Mage.
I couldn't find any specific mechanics for how something becomes plaguechanged, like a disease track for progressing the plague to somewhere irrevocable, or a way for aboleths to benefit from a single new recruit. There are some plot sketches of the ultimate goal they're poking at, and of course, anybody the PCs care about is probably better off not leaking blue fire from the eyes and whispering madness, all told.
There are also mechanics for making plaguechanged creatures.
The Spellplague distorts everything it encounters - stone, flesh, spirit. Creatures infested by its corruptive influence, known as plaguechanged, manifest bizarre arcane powers.
-- Neverwinter Campaign Setting, p.95
Specifically there's a plaguechanged monster template, with links to other templates that provide powers which are also in-theme. So pick a base monster that fits what you want out of an encounter, slot a couple plaguechanged powers into it, and now you've got a mad guard captain who can mark you for arrest with a spellfire gaze and break free of containment by turning into a volatile fireball.