One glance at a beholder is enough to assess its foul and otherworldly nature. Aggressive, hateful, and greedy, these aberrations dismiss all other creatures as lesser beings, toying with them or destroying them as they choose.
A beholder's spheroid body levitates at all times, and its great bulging eye sits above a wide, toothy maw, while the smaller eyestalks that crown its body twist and turn to keep its foes in sight. When a beholder sleeps, it closes its central eye but leaves its smaller eyes open and alert.
Xenophobic Isolationists. Enemies abound, or so every beholder believes. Beholders are convinced that other creatures resent them for their brilliance and magical power, even as they dismiss those lesser creatures as crude and disgusting. Beholders always suspect others of plotting against them, even when no other creatures are around.
The disdain a beholder has for other creatures extends to other beholders. Each beholder believes its form to be an ideal, and that any deviation from that form is a flaw in the racial purity of its kind. Beholders vary greatly in their physical forms, making conflict between them inevitable. Some beholders are protected by overlapping chitinous plates. Some have smooth hides. Some have eyestalks that writhe like tentacles, while others' stalks bear crustacean-like joints. Even slight differences of coloration in hide can turn two beholders into lifelong enemies.
Eye Tyrant. Some beholders manage to channel their xenophobic tendencies into a terrible despotism. Rather than live in isolation, the aptly named eye tyrants enslave those other creatures, founding and controlling vast empires. An eye tyrant sometimes carves out a domain within or under a major city, commanding networks of agents that operate on their master's behalf.
Alien Lairs. Because they refuse to share territory with others, most beholders withdraw to frigid hills, abandoned ruins, and deep caverns to scheme. A beholder's lair is carved out by its disintegration eye ray, emphasizing vertical passages connecting chambers stacked on top of each other. Such an environment allows a beholder to move freely, even as it prevents intruders from easily creeping about. When intruders do break in, the height of its open ceilings allows a beholder to float up and harry foes on the floor.
As alien as their creator, the rooms in a beholder's lair reflect the creature's arrogance. It festoons its chambers with trophies from the battles it has won, including petrified adventurers standing frozen in their horrified final moments, pieces of other beholders, and magic items wrested from powerful foes. A beholder judges its own worth by its acquisitions, and it never willingly parts with its treasures.