The beholder is among the most classic of all Dungeons & Dragons monsters, appearing in every edition of the game since 1975. Beholders are one of the few classic Dungeons & Dragons monsters that Wizards of the Coast claims as Product Identity, which is the reason why their stats are not part of any Open Game Content.
Physical Description and Powers
A Beholder is an aberration comprising a floating spheroid body with a large fanged mouth and single eye on the front and many flexible eyestalks on the top. Its eyes each possess a different magical ability; the main eye projects an anti-magical cone, and the other eyes use different effects, such as disintegrating objects, transmuting flesh to stone, causing sleep, slowing the motion of objects or beings, charming animals and/or humans, causing death, inducing fear, levitating objects, and inflicting serious wounds. Many variant beholder species exist, such as «observers», «spectators», «eyes of the deep», «elder orbs», «hive mothers», and «death tyrants». In 4e, different breeds of Beholders have different magic abilities, alternating energy rays encompassing fire, frost and necrotic.
Beholders are extremely xenophobic. They will sometimes take members of other, non-beholder races as slaves; however, they will engage in a violent intra-species war with others of their kind who differ even slightly in appearance. This intense hatred of other beholders is not universal; the most prominent exceptions are Hive Mothers, who use their powers of mind control to form hives with other beholders and beholder-kin. Beholder communities in the Underdark often, when provoked, wage war on any and all nearby settlements, finding the most resistance from the drow and illithids. Beholders worship their insane, controlling goddess known as the Great Mother.
According to Keith Baker in his «Eberron Expanded» column about the Lords of Madness sourcebook, «beholders served as living artillery during the Daelkyr incursion, using the terrible power of their eyes to shatter whole goblin armies. In Eberron, beholders do not reproduce naturally and have not created a culture of their own—they are simply the immortal servants of the daelkyr. Most continue to serve their masters, commanding subterranean outposts of aberrations or serving as the hidden leaders of various Cults of the Dragon Below. Others lead solitary lives, contemplating mysteries or studying the world. Such lone beholders may manipulate humanoid communities, but their actions are rarely driven by a desire for personal power. Members of the Cults of the Dragon Below believe that these creatures function as the eyes of a greater power. Some insist that they serve Belashyrra, a powerful Daelkyr who is also known as the Lord of Eyes. Others claim the beholders are the eyes of Xoriat itself—that while they serve the daelkyr, they are conduits to a power even greater and more terrible than the shapers of flesh.»
In your campaign
Just as a finishing line, I’d like to add that whatever use you give to beholders in your campaign, just don’t have them as easily-trickable watching dogs as they did in the first Dungeons & Dragons movie. Beholders work better in their lair (5e has them as legendary monsters) so excel at what they do best: terrorize your party giving signs they will find one… and make it appear when they least expect it.