Beholder

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 region containing a beholder’s lair is warped by the creature’s unnatural presence. Marks on cave walls might change subtly, an eerie trinket might appear where none existed before, harmless slime might coat a statue, and so on.