I'll do 1st ed, 'cause I'm a grognard. I'm sure someone can do a conversion.
MONSTER UNDER THE BED
FREQUENCY: Common
NO. APPEARING: 1
ARMOR CLASS: 3
MOVE: 12" (limited to area under bed)
HIT DICE: 8
% IN LAIR: 100%
TREASURE TYPE: C
NO. OF ATTACKS: 1
DAMAGE/ATTACK: 4-16 + grab
SPECIAL ATTACKS: grab
SPECIAL DEFENSES: invisible in light or to adults, ethereal
MAGIC RESISTANCE: 30%
INTELLIGENCE: High
ALIGNMENT: Neutral evil
SIZE: Variable (S to M)
PSIONIC ABILITY: Nil
The monster under the bed is known to terrorize children of all sorts, but invariably disappears when the light is switched on or when parents arrive. Of course, this is due to the monster's unusual enchantment that allows it to remain unseen by any adult as well as in any sort of light, and it is believed to possibly be a relative of the invisible stalker (q.v.). Under the influence of detect invisibility or similar spells a tentacled, grasping, amorphous mass is often seen, but reports vary.
Children wisely remain on the bed when they hear the monster prowling, for to leave a limb in the monster's pathway between the bed and floor is to risk being grabbed and eaten. The grab is treated as a common attack roll, and if it hits, the target must make a Strength roll or be grabbed by the monster and dragged underneath to be eaten. If it survives the initial attack, it may make a Strength roll each round to break out of the monster's grip, though the monster's attack hits automatically every round it has its prey grabbed. The monster is able to shift as a phase spider (q.v.) into the ethereal, which it tends to do whenever threatened; it will usually drop its prey if it does this.
Despite the ubiquity of such monsters they rarely attack (usually on a roll of 20 on a d20), possibly being occupied on whatever plane they occupy most of the day, likely explaining the low number of fatalities due to this creature.