Its primary abilities include a melee for approximately 2K on plate, a random AoE fear (which is very likely to cause adds in a populated zone), and a channeled beam that inflicts 400-500 damage per second of random types.
Collidus the Warp-Watcher is a criterion of the achievements:
"The beholder is a fictional monster in the Dungeons & Dragons fantasy role-playing game. It resembles a floating orb of flesh with a large mouth, single central eye, and lots of smaller eyestalks on top with deadly magical powers.
The Beholder is among the most classic of all Dungeons & Dragons monsters, appearing in every edition of the game since 1975. They are one of the few classic Dungeons & Dragons monsters that Wizards of the Coast claims as Product Identity."(Info gained from Wikipedia)