Over the course of centuries, psionically enhanced sites can gain more power, either in latent psionic power from those who come to the site, or intentionally from its creators or those who would see the site grow greater in effect. If the power grows great enough, the very site can gain a sentience of its own, becoming aware of both itself and those who come in contact with the sentient region.
Just as with sentient creatures, sentient regions have their own individual personalities, although the very nature of a sentient region tends to make their views on concepts hard to grasp by most others. The nature of a sentient region tends to draw on the purpose for its awakening and its initial creation. If the site was used for meditation and peaceful activities, the region itself tends to reflect this view, while those sites devoted to combat tend to be more aggressive. Sentient regions originally created for evil purposes are often sinister sites that inspire the darkest of nightmares: caves that attempt to devour those who enter, trees that intentionally crack branches to catch passers-by unaware, or streams with an unusually strong and deadly undertow.
Two centuries ago, a band of maenads were migrating through a thick forest when they chanced upon a small glen, granting them a spot to rest for the night. Unbeknownst to the maenads, the glen was an imbued site created by a forgotten group of goblins, imbued to inspire those who rested within to slaughter and combat. During their sleep, the maenads were overcome by the persistent suggestions implanted by the glen, ending in the death of every maenad in the group. What started as an imbued site for battle and slaughter grew over time as the blues and goblins continued to journey to the glen. Eventually, the glen turned on its creators, causing them to slaughter each other and themselves. Any who stay within the bounds of the glen find it difficult to resist its urgings and invariably find themselves committing acts they would never normally perform.
Those who enter Nightmare Glen initially feel slightly uneasy, although they cannot place the reason why. If an individual or group of creatures spends more than one hour in the glen, they begin to feel the effects of the sentience present. Initially, this is manifested as becoming agitated and easily provoked. After another hour, the affected creatures react to all creatures in one step worse than they normally (so a friendly reaction becomes indifferent). Two hours after this, they advance another step worse in reaction and are likely to be provoked into combat. Every ten minutes, the affected character must make a successful Will save against a DC of 15 or become violent, attacking the nearest target.