(Originally from http://dngnsndrgns.blogspot.co.uk/2013/08/d20-icons-beholder.html)
Level 6 (Triple Strength Large Aberrant Magical Beast)
HP: 250 AC: 21 PD: 18 MD: 20
Bite : +11 vs AC - 55 damage.
Miss : 22 damage
Central Eye : (once per round as a quick action)
+11 vs. MD - The target is Stunned save ends.
Eye Stalks : (2 attacks from the following list, attacks do not provoke attacks of opportunity and can only be made against nearby creatures)
Roll a d10 or choose:
Charm Ray: +11 vs. MD - The target is Confused (save ends)
Wounding Ray: +11 vs. PD - 63 damage, Miss: 31 damage
Sleep Ray: +11 vs. MD - The target is Stuck (save ends) First failed save, the target is Helpless (save ends)
Telekinesis Ray: +11 vs. PD - The beholder forces the target to move as a free action.
Slowing Ray: +11 vs. PD - 40 damage and the target is Stuck (save ends)
Brilliant Ray: +11 vs. PD - 23 damage and the target is Hampered. (save ends
Terror Ray: +11 vs. MD - 23 damage and the target is affected by Fear (save ends)
Petrifying Ray: +11 vs. PD - The target begins taking last gasp saves as it turns to stone.
Death Ray: +11 vs. PD - 36 damage, if the target is staggered they are also Stunned. The target also starts making last gasp saves as they begin to die!
Disintegrate Ray: +11 vs. PD - 40 damage and 20 ongoing damage.
Sudden realisation of the concept of mortality: When first staggered the Beholder uses its Eye Stalks, but with 3 attacks as a reaction. Every time the Beholder uses its Eye Stalks and is staggered it makes 3 attacks instead of 2.
Aim for the eyes!: When an attack hits the Beholder, if the roll is a natural 18-20 choose one Eye stalk attack randomly, the Beholder cannot use this attack for the remainder of the battle.
How dare you approach me worm!: If an enemy creature begins its turn nearby the Beholder, the Beholder uses 1 Eye Stalk attack against it as a free action.
The beholder is a real nasty chap, much like dragons in the core book he has the real potential to destroy a party, so i'd let them know how serious the battle is and definitely give them the option to retreat (at a cost of course!)