I'll try not to spoil, but just nudge you in the right direction...
Have you been inside the temple at the area where you find all the diseased people? There's something in there that needs an unconventional tactic to defeat (read what the in-game description says for a hint). Once that is dealt with an avatar of the temple's god will appear. Talk to it & it'll gove one half of the rift device.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc