All-Seer

Amorphous. The All-Seer can move through a space as narrow as 1 inch wide without squeezing.

Spider Climb. The jelly can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Insanity. The All-Seer is immune to any spell that would affect its thoughts, such as detect thoughts or modify memory,  and it must randomly decide what movement type it will use on a turn. It must decide this at the beginning of the turn, and it can chose not to move at all afterwards. 

d4             Result

1                walk

2                climb

3                fly

4                the All-Seer choses

Pseudopod. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) bludgeoning damage plus 3 (1d6) acid damage.

Madness blast. The All-Seer uses it's telepathy to communicate it's ravaged sanity to a creature within the range of it's telepathy. The creature must make a DC 17 wisdom saving throw or take 7d6 psychic damage or half as much on a successful save. 

Eye Rays. The All-Seer shoots three of the following magical eye rays at random (reroll duplicates), choosing one to three targets it can see within 120 ft. of it:

1. Hallucination Ray. The targeted creature must succeed on a DC 17 wisdom saving throw or take 2d6 psychic damage at the start of it's turn and be incapacitated. It can make the saving throw again at the start of each of it's turns, before it takes the damage. 
2. Paralyzing Ray. The targeted creature must succeed on a DC 16 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
3. Fear Ray. The targeted creature must succeed on a DC 16 Wisdom saving throw or be frightened for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
4. Slowing Ray. The targeted creature must succeed on a DC 16 Dexterity saving throw. On a failed save, the target's speed is halved for 1 minute. In addition, the creature can't take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
5. Enervation Ray. The targeted creature must make a DC 16 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one.
6. Telekinetic Ray. If the target is a creature, it must succeed on a DC 16 Strength saving throw or the beholder moves it up to 30 ft. in any direction. It is restrained by the ray's telekinetic grip until the start of the beholder's next turn or until the beholder is incapacitated. If the target is an object weighing 300 pounds or less that isn't being worn or carried, it is moved up to 30 ft. in any direction. The beholder can also exert fine control on objects with this ray, such as manipulating a simple tool or opening a door or a container.
7. Sleep Ray. The targeted creature must succeed on a DC 16 Wisdom saving throw or fall asleep and remain unconscious for 1 minute. The target awakens if it takes damage or another creature takes an action to wake it. This ray has no effect on constructs and undead.
8. Petrification Ray. The targeted creature must make a DC 16 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.
9. Disintegration Ray. If the target is a creature, it must succeed on a DC 16 Dexterity saving throw or take 45 (10d8) force damage. If this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust. If the target is a Large or smaller nonmagical object or creation of magical force, it is disintegrated without a saving throw. If the target is a Huge or larger object or creation of magical force, this ray disintegrates a 10-foot cube of it.
10. Death Ray. The targeted creature must succeed on a DC 16 Dexterity saving throw or take 55 (10d10) necrotic damage. The target dies if the ray reduces it to 0 hit points.

Split. When an All-Seer that is Medium or larger is subjected to lightning or slashing damage, it splits into two new All-Seers if it has at least 10 hit points. Each new All-Seer has hit points equal to half the original All-Seer's, rounded down. New All-Seer's are one size smaller than the original All-Seer. A new All-Seer has the ability to use half of the eye rays, rounded down, of it's "parent". That number can be zero. 

The All-Seer can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time, and only at the end of another creature's turn. The All-Seer regains spent legendary actions at the start of its turn.

Eye Ray. The All-Seer uses one random eye ray.