Kenku, humanoid monsters that inhabited the Underdark, were not particularly powerful in their own right, with a Challenge Rating of only "1/2."
Normally, any proper party of professionals should have been in no danger of coming to grief against such monsters.
In practice, however, Kenku were among the monsters adventurers least wanted to encounter in the Underdark.
The main reason lay in their special ability, "Terrifying Screech."
After a preparatory motion, a Kenku unleashed a piercing blast of sound, forcing every creature other than Kenku within a sixty-foot radius reached by the screech to make a Perception saving throw.
Success meant nothing happened; failure left the affected creature with restricted movement and spellcasting for a period of time. Not only could they no longer move quickly or climb, but their spellcasting abilities were also restricted to below the First Circle.
The screech of a single Kenku might have been just barely bearable.
But Kenku were famously communal creatures in the Underdark.
Like migratory birds on the surface and herbivores on the grasslands, their racial instincts often drove Kenku to gather into enormous flocks, roaming and ravaging the tunnels and caverns of the Underdark in particular patterns.
Sometimes, even powerful monsters whose Challenge Ratings alone could make low-level adventurers wet themselves would not dare show their faces when a vast flock of Kenku passed their doorstep. They simply huddled quietly in their lairs to avoid
inviting trouble.
Xia Nan might have possessed outstanding Perception, allowing him to easily resist the "Terrifying Screech" of one or two ordinary Kenku.
But when their numbers reached dozens or even hundreds, even if his Perception Attribute bonus reduced his chance of being affected to one percent, so many successive assaults still left the possibility of a failed save.
Not to mention the amplified power produced