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A Guu is a Fallen in Oninaki.

Horrific Fallen whose bodies are dominated by a grotesque maw. They summon other Fallen out of thin air, so proritize defeating these first.
Research Log 1-2
According to my associate, the temperature inside the Fallen was quite agreeable, even relaxing. There was also, however, an intense, intoxicating odor, leaving it up for debate as to how much of this experience was real or hallucinated.